MPLX issues a Schedule K-1 (Form 1065) to every unitholder each year instead of the Form 1099-DIV that stock investors receive. The K-1 reports your share of the partnership’s income, deductions, credits, and cash distributions, and the numbers on it feed into more than one line of your return. If you own MPLX units, the K-1 tax form is the document your entire tax picture for that investment runs through.
Why You Get a K-1 and Not a 1099
MPLX is a master limited partnership. Federal law lets certain partnerships whose income comes mainly from natural resources, pipelines, and similar qualifying activities trade on public exchanges without being taxed as corporations.1Office of the Law Revision Counsel. 26 U.S. Code 7704 – Certain Publicly Traded Partnerships Treated as Corporations The partnership itself pays no federal income tax. Its income, gains, losses, and deductions flow through to unitholders, who report their proportional share on their own returns.
The IRS treats you as a limited partner, not a shareholder. So you get a Schedule K-1 that breaks out your slice of the partnership’s return rather than a 1099-DIV summarizing dividends.2Internal Revenue Service. About Form 1065, U.S. Return of Partnership Income
What’s Reported on the K-1
The K-1 is organized into numbered boxes, each tied to a category of partnership activity. The ones MPLX unitholders most often deal with are:
- Box 1, ordinary business income or loss from the partnership’s operations.
- Box 2, net rental real estate income or loss.
- Box 5, interest income.
- Box 19, distributions of cash and marketable securities you received during the year.
These come directly from the official K-1 form.3Internal Revenue Service. Schedule K-1 (Form 1065) – Partner’s Share of Income, Deductions, Credits, etc. You’ll want to cross-reference the figures against your own records of what you paid for your units and when you bought them, because those numbers feed cost basis calculations you’ll need at sale.
When the K-1 Arrives
MPLX posts its K-1 tax packages online and mails printed copies in mid-to-late March.4MPLX Investor Relations. Investor Data For the 2025 tax year, MPLX made packages available online with physical mailings beginning March 23, 2026. You can pre-register on the MPLX Tax Package Support website to be notified when the forms are posted.
Because K-1s often arrive later than other tax documents, many MLP investors file Form 4868 to extend their filing deadline to October 15. An extension gives you more time to file but does not extend the deadline for paying. If you expect to owe, estimate and pay by the original April deadline to avoid interest.
Distributions Reduce Your Cost Basis
This is where MLPs diverge most sharply from ordinary stock. The quarterly cash distributions MPLX sends you are not dividends. They are treated as a return of capital, so each distribution lowers your cost basis in the units rather than being immediately taxed as income.
If you bought units at $30 and received $2 in distributions during the year, your adjusted basis drops to $28. As long as basis stays above zero, distributions are tax-deferred. Once distributions have taken your basis down to zero, every dollar after that is taxable as a capital gain in the year received. Distributions are reported through Box 19.3Internal Revenue Service. Schedule K-1 (Form 1065) – Partner’s Share of Income, Deductions, Credits, etc. Track your adjusted basis year over year. Without it, you cannot compute an accurate gain when you sell.
What Happens When You Sell Units
Selling MPLX units is not simply sale price minus purchase price. Distributions have been reducing your basis for the entire holding period, so the taxable gain is usually larger than the raw price change suggests. And not all of that gain gets favorable capital gains treatment.
Depreciation Recapture
MLPs like MPLX own depreciable assets such as pipelines and storage facilities. Your share of that depreciation reduced your taxable income each year and also lowered your cost basis. When you sell, the IRS recaptures the benefit. The portion attributable to real property depreciation is taxed at a maximum federal rate of 25% as unrecaptured Section 1250 gain. Recapture on personal property under Section 1245 is taxed at your ordinary income rate, which can reach 37% for high earners.
Ordinary Income From Hot Assets
Under IRC Section 751, certain partnership assets classified as unrealized receivables or substantially appreciated inventory are treated as “hot assets.” The portion of your sale proceeds attributable to these items is taxed as ordinary income regardless of how long you held the units. The partnership reports the information needed to make this calculation on your K-1, but the math is often difficult to work through without professional help.
Additional Taxes and Filings Your K-1 Can Trigger
Net Investment Income Tax
Unitholders whose modified adjusted gross income exceeds statutory thresholds owe an additional 3.8% Net Investment Income Tax on some or all of their partnership income. The thresholds are $200,000 for single filers, $250,000 for married couples filing jointly, and $125,000 for married filing separately.5Office of the Law Revision Counsel. 26 USC 1411 – Imposition of Tax These figures do not adjust for inflation, so more investors cross the line each year. If you’re above the threshold, you’ll report the tax on Form 8960.6Internal Revenue Service. About Form 8960, Net Investment Income Tax Individuals, Estates, and Trusts
State Returns in States You’ve Never Lived In
MPLX operates pipeline, storage, and logistics assets across multiple states, and each of those states may tax your share of the income sourced there. Most states require nonresident partners to file a return and pay tax on their share of partnership income, though some offer a de minimis threshold. Rates and thresholds vary.
Some states allow the partnership to file a composite return covering nonresident investors, which spares you from filing separately in that state. Whether MPLX makes this election varies by state and year. Information about composite filings and state-specific K-1 data is typically included in the tax package MPLX provides. For many unitholders the practical effect is three to six additional state returns.
MPLX in an IRA or Other Retirement Account
Holding MPLX inside a tax-advantaged account does not automatically shield you from tax. When an IRA earns income from a partnership that operates a trade or business, the IRS treats that income as unrelated business taxable income. If gross UBTI from the partnership reaches $1,000 or more in a year, the IRA’s custodian must obtain a separate employer identification number for the account and file Form 990-T, and the account itself owes tax on that income.7Internal Revenue Service. Instructions for Form 990-T (2025) The tax is paid out of the IRA’s assets rather than your personal funds, but it still erodes the tax-deferred compounding you were counting on. Check your K-1 carefully if MPLX sits inside a retirement account.
The Section 199A Deduction Heading Into 2026
Through the 2025 tax year, unitholders in publicly traded partnerships could claim a deduction of up to 20% of qualified business income from the partnership under Section 199A.8Internal Revenue Service. Qualified Business Income Deduction The provision was enacted by the Tax Cuts and Jobs Act and was scheduled to expire on December 31, 2025.9Congress.gov. Expiring Provisions in the Tax Cuts and Jobs Act (TCJA, P.L. 115-97) As of early 2026, legislation to extend or make the deduction permanent has been introduced in Congress but has not been enacted. If it is not extended, MPLX income for the 2026 tax year will be taxed at full ordinary income rates without the 20% offset.
If You Get a Corrected K-1
Partnerships sometimes discover errors after the initial K-1 has gone out. If MPLX issues a corrected K-1, do not alter your original copy. Contact the partnership to confirm the correction and make sure a copy of the corrected form goes to the IRS.10Internal Revenue Service. Partner’s Instructions for Schedule K-1 (Form 1065) If you already filed, you’ll likely need to amend. And if you disagree with how the partnership reported an item on your K-1, you must file Form 8082 with your return to flag the inconsistency. Changing the number on your return without disclosing the disagreement invites an IRS notice.