2025 MPLX K-1 Guide: Marathon's MLP With Growing Basis Erosion

MPLX LP is a large-cap midstream MLP formed by Marathon Petroleum Corporation (MPC), one of the largest independent refiners in the U.S. Marathon is both MPLX's general partner and majority unitholder — giving MPLX access to Marathon's extensive refining and logistics network while creating a built-in growth pipeline through "dropdown" transactions (Marathon contributing midstream assets to MPLX in exchange for units).

Marathon Petroleum Corporation (MPC) owns the general partner interest in MPLX and is its majority unitholder. MPC is a C-corporation — it issues a 1099-DIV. MPLX is the limited partnership that holds the midstream assets — it issues a K-1.

Investors who own BOTH MPC stock and MPLX units receive different tax treatment: MPC dividends are taxed as qualified dividends, while MPLX distributions reduce your cost basis and trigger K-1 reporting, §751 recapture at sale, and potential multi-state filing. This is the same corporate/MLP pattern as PAA/PAGP and CQP/LNG.

MPLX's distribution yield has historically been in the 8–10% range — higher than EPD and many other large midstream names. For basis tracking purposes, this is a double-edged sword.

• More cash in your pocket: Higher quarterly distributions mean more income each year
• Faster basis reduction: Most of the distribution is return of capital (Box 19A exceeds Box 1 income), which reduces your tax basis dollar-for-dollar
• Larger gain when you sell: Lower basis = more taxable gain on disposition, even if the unit price hasn't changed

Enter your MPLX K-1 data to calculate your adjusted basis for one year.

MPLX's K-1 includes the following key items. The exact amounts depend on your unit count and holding period:

• Box 1 — Ordinary Business Income (Loss): Usually a net loss due to heavy depreciation deductions from MPLX's pipeline and processing asset base. This loss is suspended under §469(k) PTP passive rules and released when you sell.
• Box 19A — Distributions: The cash you received during the year. This is the big number — and it reduces your basis dollar-for-dollar.
• Item K — Partner's Share of Liabilities: Beginning and ending liability figures. Liability increases raise your basis; decreases lower it. Don't skip this line — it directly impacts the worksheet.
• Box 20, Code Z — §199A QBI Information: Qualified business income data for the 20% QBI deduction. Check the supplemental statement attached to the K-1 — the actual data isn't in the box itself.
• Sale year — Sales Schedule (§751 recapture): In the year you sell, your final K-1 package includes a Sales Schedule showing the §751 ordinary income recapture amount. This is reported on Form 4797, not in Box 20.

If you own both MPLX and Energy Transfer, here's how their tax obligations differ:

MPLX operates primarily in the Appalachian Basin (Pennsylvania, West Virginia, Ohio), the Permian Basin (Texas, New Mexico), and along the Gulf Coast. Your K-1 state schedule allocates income to each state where MPLX does business.

States to watch: Pennsylvania and West Virginia are major MPLX operating states with relatively low nonresident filing thresholds. Ohio has its own pass-through entity rules.

Texas: No state income tax — MPLX's significant Texas operations don't create individual filing requirements.

Practical approach: Most MPLX investors with positions under 500 units will file in their home state plus 0–2 additional states. As with all MLPs, the year you sell expands state filing significantly. See our state filing requirements guide for the full framework.

From an institutional allocation standpoint, MPLX's Marathon affiliation is both the primary appeal and the key risk factor.

Competitive advantage: Marathon Petroleum dropdown transactions give MPLX a captive growth pipeline that most standalone MLPs don't have. When Marathon acquires or develops midstream assets, they eventually flow to MPLX — providing visibility on future distribution growth. MPLX's 9.4% distribution CAGR produces dramatically different 20-year outcomes than slower-growing MLPs; see the full 20-year holding analysis for the year-by-year tax math.

Sponsor risk: This same relationship creates concentration risk. If Marathon's strategy shifts (as it did with the 2019 activist campaign that led to Speedway's sale), MPLX's growth outlook changes immediately. The partnership can't diversify its sponsor.

Appalachian exposure: MPLX's gathering and processing operations in the Marcellus/Utica shale region give it leveraged exposure to natural gas production growth in the most prolific gas basin in the U.S. As LNG export capacity expands, Appalachian production should follow — benefiting MPLX's volume-based contracts.

• Assuming "breakeven" means no tax: If MPLX's unit price hasn't changed since you bought, you might think there's no gain. But basis erosion means you have a taxable gain equal to the cumulative basis decline. Selling at "breakeven" on price ≠ breakeven on taxes. → Calculate your real MPLX basis free
• Not checking "Publicly Traded Partnership": In TurboTax, this checkbox determines §469(k) PTP passive activity treatment. Without it, losses may incorrectly offset other income. → PTP passive loss rules
• Missing the §199A supplemental statement: The §199A QBI data isn't in Box 20 itself — it's in an attached statement. If your tax software only reads the box number, you'll miss the deduction. → Basis Worksheet Explained
• Ignoring liability changes (Item K): Liability increases are treated as basis contributions; decreases are treated as distributions. This line is easy to overlook but directly impacts your worksheet. → K-1 Basis Tracker tracks this automatically
• Using broker basis when selling: Your 1099-B shows the purchase price. The IRS expects K-1-adjusted basis. Override on Form 8949 with code B in column (f) — indicating the broker's reported basis is incorrect — and your corrected basis in columns (e) and (g).
• Confusing MPC (1099-DIV) with MPLX (K-1) tax treatment: Marathon Petroleum stock (MPC) issues a 1099-DIV. MPLX units issue a K-1. They are separate securities with fundamentally different tax reporting. If you own both, make sure you're tracking MPLX's K-1 basis separately.