Form 637 Registration: Activity Letters, Documents, and Filing

To apply for Form 637 registration, identify the excise tax activity letters that match your operations, assemble the entity, ownership, operational, and financial documentation the IRS wants to see, and send the completed form with its attachments to the IRS Excise Operations Unit in Cincinnati by mail or fax. The agency runs each application through activity-specific registration tests, targets 45 days for a clean file, and only issues tax-free transaction authority once you receive an official Letter of Registration. A submitted application in the queue gives you nothing.

Confirm You Need to Register Before You Start

Form 637 is for businesses that manufacture, produce, import, or sell products subject to federal excise tax and want to engage in tax-free transactions or claim excise credits and refunds. Fuel activities fall under IRC Section 4101, which covers handlers of taxable fuels, producers of biodiesel or alcohol fuels, and producers of sustainable aviation fuel and other clean fuels.¹ Manufacturer and retailer excise activity falls under Section 4222, which requires registration before a manufacturer, first purchaser, or second purchaser can buy or sell tax-free under Section 4221.² Registration also applies to taxable chemicals and ozone-depleting chemicals under Sections 4662 and 4682.³

One point worth being blunt about: the IRS must approve the registration before you get any benefit from it. Operating without registration carries a $10,000 penalty for each initial failure under IRC Section 6719, plus $1,000 for every day the failure continues. Waiting until an audit surfaces the issue is expensive.

Pick the Right Activity Letters

The form is organized around activity letters, each tied to a specific role in the supply chain of a taxable product. You mark the letters that match what your business actually does, and each one carries its own documentation requirements and registration tests. Mismatched codes are one of the fastest ways to have an application delayed or denied, so this step is where the application really starts.

Common fuel-related codes under Section 4101 include:

Letter S for terminal operators, enterers, position holders, and refiners of taxable fuels — the parties responsible for the first taxable removal or entry of fuel into the distribution system.
Letter M for blenders of gasoline, diesel fuel, or kerosene.
Letters AB and AF for producers and importers of agri-biodiesel and alcohol fuels.
Letters AL and AM for alternative fuelers selling or using alternative fuel in motor vehicles, or producing alternative fuel mixtures for their trade or business.
Letters CA and CN for producers of clean transportation fuel, with CA covering sustainable aviation fuel and CN covering other clean fuels.

Outside fuel, other codes cover different excise regimes:⁴

Letter A for manufacturers of gas guzzler automobiles, sport fishing equipment, taxable tires, bows, arrow components, or vaccines.
Letter B for buyers of those items for further manufacturing or resale to another manufacturer.
Letter D for buyers purchasing taxable articles for export.
Letter G for persons making inventory exchanges of taxable chemicals or dealing in intermediate hydrocarbon streams.
Letter CC for credit card issuers handling fuel sales to state and local governments or nonprofit educational organizations.

The form has over 20 activity letter codes in total, and a single business can register for multiple activities on one Form 637 if its operations span several roles. Working from the complete chart on the form itself is the safest way to confirm which codes apply.⁵

Gather Your Documentation

Form 637 asks for far more detail than a typical IRS form. Assemble everything below before you begin filling it out.

Entity and Ownership

Provide your legal business name, Employer Identification Number (EIN), and every current operating address. The IRS also wants a full ownership picture: names, Taxpayer Identification Numbers, and ownership percentages for every owner, officer, or partner. Parent companies, subsidiaries, and related entities need to be disclosed with each related entity’s EIN. Include your articles of incorporation, partnership agreement, or equivalent formation document, along with any state and local licenses tied to the activities you are registering for.

Business Activity

Write a thorough description of your operations, including when the relevant activity started or will start, and explain exactly how your business fits into the supply chain of the taxable product. Identify the organizations you plan to sell tax-free articles to, and give estimated monthly volumes of both taxable and tax-free products.

Financials

Financial capacity to cover excise liabilities matters, especially for fuel applicants. Expect to attach a copy of your most recent federal income tax return, a current income statement, a balance sheet, or an independent bond rating. The reviewer uses these to evaluate whether your business can pay the excise taxes it will owe.

Bond Requirement for Fuel Applicants

If you are registering for fuel activities under Section 4101, the IRS can require you to post a surety bond as a condition of registration.¹ The bond guarantees payment of excise taxes on gasoline, diesel fuel, or kerosene moving through your operations, and you file it on Form 928 (Taxable Fuel Bond) separately from Form 637.⁶

The bond amount is not fixed. The IRS sets it based on your financial capabilities, tax history, and expected liability, and it cannot exceed your expected excise tax liability under Sections 4041(a)(1) and 4081 for a representative six-month period. Terminal operators get an additional calculation based on expected tax liability from fuel removed at their terminal racks during a representative one-month period.⁶

The surety company writing the bond must be listed in Treasury Department Circular 570, the federal government’s directory of approved sureties and reinsurers.⁶ Annual premiums for federal fuel excise bonds generally run between 1% and 15% of the total bond amount, with rates driven mostly by your credit profile. Instead of a traditional surety bond, you can post U.S. Treasury notes or bills as collateral, or provide an irrevocable letter of credit from an approved financial institution.

Where to Send the Application

Once Form 637 is complete with all selected activity letters and every supporting document assembled, mail or fax the entire package to the IRS. The mailing address is:

Department of the Treasury
Internal Revenue Service Center
Excise Operations Unit – Form 637
Mail Stop 5701G
Cincinnati, OH 45999⁵

The fax number is 855-887-7735.⁵ There is no online submission option. If you fax, keep the confirmation page as proof of submission.

How the IRS Reviews Your Application

Applications are not rubber-stamped. Each one runs through registration tests that vary by activity letter, and the reviewer checks whether you meet the standards for every activity you requested.⁵

Internal IRS guidelines target 45 days for the compliance review of a clean initial application. Deficiencies in the paperwork extend the timeline to 60 days. When third-party contacts are needed to verify your information, it can stretch to 120 days from assignment.⁷ Some applications take longer in practice, so do not schedule tax-free transactions around an assumed approval date.

Part of the review may include an unannounced visit to your business premises during normal business hours. A revenue agent will inspect your facilities to verify that your described operations are legitimate and consistent with the activity letters you requested. Operations that do not match the application often lead to denial. For applicants that pose potential revenue risk, the IRS also performs a financial analysis covering expected excise tax liability, available cash, profit margins on taxable products, and competing expenses like debt payments and salaries.⁸

When approved, the IRS issues a Letter of Registration specifying your registered activities, effective date, and unique registration number. A copy of your submitted Form 637 is not a substitute for the letter and cannot be used to conduct tax-free sales.

If Your Application Is Denied

A denial arrives as Letter 3685 with a written explanation.⁷ You then have 10 days from the postmark date on that letter to submit a written appeal. Miss the window and you lose the appeal and must start over with a new application.

When an appeal is filed on time, a lead examiner reviews the case and has 14 days to make a determination. If the reviewer agrees the registration should proceed, the process continues. If the reviewer upholds the denial, the case moves to a separate contested-denial review process. The same 10-day appeal window applies if the IRS later revokes an existing registration, using Letter 3691 instead of 3685.⁷

Denials most often come from incomplete applications, inability to demonstrate financial responsibility, unresolved federal tax debts, or operations that do not match the requested activity letters. Fixing those before you submit is far cheaper than fixing them after.

1
Office of the Law Revision Counsel. 26 U.S. Code 4101 – Registration and Bond
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Office of the Law Revision Counsel. 26 U.S. Code 4222 – Registration
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Internal Revenue Service. About Form 637, Application for Registration (For Certain Excise Tax Activities)
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Internal Revenue Service. IRS Form 637 – Application for Registration (For Certain Excise Tax Activities)
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Internal Revenue Service. 637 Registration Program
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Internal Revenue Service. Form 928 – Instructions for Form 928, Taxable Fuel Bond
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Internal Revenue Service. Form 637 Registration Files – Administrative Procedures
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Internal Revenue Service. Operational Procedures and Responsibilities for Form 637