Form 637 Excise Tax Registration Guide

Much of the federal excise system runs on registration. Sell fuel tax-free into the terminal system, claim vendor refunds, or produce clean transportation fuel for the section 45Z credit, and the IRS wants you vetted first. Form 637 is the gateway: one application, dozens of activity letters, each unlocking a specific role in the excise ecosystem.

Here is who must register, what the letters mean, how the vetting works, and how to check anyone’s registration — including your own — in the IRS’s online tool.

Who MUST register

The Form 637 instructions list the mandatory registrants:

Pipeline or vessel operators within the bulk transfer/terminal system — activity letter X
Enterers, position holders, refiners, and terminal operators — activity letter S
Blenders of gasoline, diesel, or kerosene — activity letter M
Producers or importers of alcohol, agri-biodiesel, and biodiesel (including renewable diesel) — letters AF, AB, and NB
Producers of clean transportation fuel — letter CA for sustainable aviation fuel, CN for non-SAF fuel (the section 45Z letters)
Producers or importers of SAF (letter SA) and producers of second generation biofuel (letter SB)

The penalty, verbatim from the form: "The penalty for failure to register, if you’re required to register, is $10,000 for the initial failure, and $1,000 for each day following that you fail to register" — unless the failure is due to reasonable cause.

The refund-side letters: register or you cannot claim

A second family of letters exists for refund claimants rather than fuel handlers. Ultimate vendors selling undyed diesel, kerosene, or gasoline tax-excluded to state and local governments or nonprofit schools register as UV; vendors selling to intercity buses as UB; blocked-pump kerosene sellers as UP; aviation-kerosene vendors as UA; and credit card issuers claiming on government fuel purchases as CC.

These registrations are the price of admission to Form 8849 Schedules 2 and 8 — the schedules themselves warn: "If you do not have a registration number, you cannot make this claim." Alternative fuelers historically used letters AL and AM for the now-expired alternative fuel credits.

How the application and vetting work

Registration is not automatic. The instructions are direct: "The application must be reviewed and approved before you’re registered for any activity, and bond may be required. The review may include inspection of your premises during normal business hours without advance notice." Fuel applicants answer compliance-history questions covering penalties, fuel-tax crimes, and prior revocations — for themselves and related persons.

Approval produces a Letter of Registration listing the activities, effective date, and registration number. Two details worth flagging: "A copy of Form 637 isn’t a Letter of Registration" — the filed form proves nothing — and registrations cannot be sold, leased, or lent to anyone else. Applications go by fax to 855-887-7735 or by mail to the IRS Excise Operations Unit in Cincinnati. The IRS publishes no processing-time commitment, so build the review period into any launch plan.

After registration, changes to the information submitted must be reported within 10 days, and a more-than-50% ownership change in a fuel-activity registrant generally requires re-registration.

The 45Z wrinkle: register BEFORE you produce

For the section 45Z Clean Fuel Production Credit, timing is everything: "A taxpayer cannot claim a Clean Fuel Production Credit unless the taxpayer is registered as a producer of clean fuel at a time of production." A Form 637 filed after the fuel is made forfeits the credit for everything produced before the registration was in hand — there is no retroactive fix. Producers claim the credit on Form 7218 with their income tax return, but the CA or CN letter has to exist first.

Checking a registration online

The IRS runs a public Form 637 Registration Status Check at apps.irs.gov/app/exciseTax/ that "provides the ability for businesses to confirm whether individuals or companies have a valid IRS registration." Enter the registration number with its activity-letter suffix. Number formats vary by era: 8 digits for most older non-UV/UP registrations, 9 for UV/UP, and 10 digits for numbers issued after February 2004.

Frequently asked questions

What is Form 637 used for?⌄

Applying for IRS excise tax registration for activities under sections 4101, 4222, 4662, and 4682 — fuel blending and terminal operations, tax-free sales of taxable articles, and chemical/ozone-depleting-substance activities. Each business unit with its own EIN applies separately, and each activity letter is a separate registration.

What happens if I operate without a required registration?⌄

The statutory penalty is $10,000 for the initial failure plus $1,000 for each day the failure continues, absent reasonable cause. And for refund-side letters, the practical penalty is simpler: no registration number, no Form 8849 Schedule 2 or 8 claim.

How long does Form 637 approval take?⌄

The IRS publishes no processing-time standard for Form 637. What it does say is that every application is reviewed and approved before registration — possibly including an unannounced site inspection — so plan for a meaningful review period before you need the registration to be active.

What are the CA and CN activity letters?⌄

The section 45Z Clean Fuel Production Credit letters, added on the December 2025 form revision: CA for producers of clean transportation fuel that is sustainable aviation fuel, CN for producers of clean fuel that isn’t SAF. Registration must exist at the time of production for the fuel to generate credit.

How do I verify a supplier’s excise registration?⌄

Use the IRS’s online Form 637 Registration Status Check at apps.irs.gov/app/exciseTax/ — enter the registration number and activity-letter suffix and the tool confirms whether the registration is valid.

Related guides

Official sources

Tax rules, rates and deadlines change — verify current requirements against the IRS sources above before acting. This guide is general information, not tax or legal advice.

Form 637: IRS Excise Tax Registration, Explained