Most accounts payable mistakes don’t happen at the invoice stage. They happen earlier, when someone accepts a W-9 without actually reading it. You file a 1099 with a wrong TIN, the IRS sends a B-Notice, and suddenly you’re staring at backup withholding obligations on a vendor you’ve been paying for three years. The good news: a methodical W-9 verification review takes about ten minutes per vendor and prevents almost every downstream problem. Here’s how to do it right.
Understand What the W-9 Actually Certifies
A W-9 is not just a data collection form. When a vendor signs it, they are legally certifying under penalty of perjury that their name, taxpayer identification number (TIN), and federal tax classification are accurate. That certification shifts responsibility. If the information turns out to be wrong and you accepted it without reasonable scrutiny, the IRS can hold your business liable for backup withholding you failed to collect—currently 24% of the payment amount. So your verification is not bureaucratic box-checking. It is due diligence with a dollar sign attached.
Step 1: Match the Name to the TIN Type
Line 1 asks for the name as it appears on the vendor’s tax return. Line 2 is for a business name or disregarded entity name. The combination of name and TIN must match exactly what the IRS has on file. This is where most errors originate.
Individuals and sole proprietors
If the vendor checks “Individual/sole proprietor” in Box 3, Line 1 must show the person’s legal name—not their trade name, not their LLC name, not a nickname. Their TIN should be their Social Security Number (SSN). If they hand you a form that says “Mike’s Plumbing” on Line 1 and an EIN in the TIN box, that’s a flag. Ask them to resubmit with the owner’s legal name on Line 1 and the business name on Line 2.
Single-member LLCs
A single-member LLC that hasn’t elected corporate tax treatment is a disregarded entity. For 1099 purposes, the owner’s name goes on Line 1 and the LLC name on Line 2. The TIN should be the owner’s SSN unless the LLC has its own EIN and uses it for business purposes. This distinction trips up a surprising number of vendors—and the people processing their forms.
Corporations and partnerships
For entities that checked “C Corporation,” “S Corporation,” or “Partnership,” Line 1 should reflect the legal entity name exactly as registered. The TIN here is always an EIN. If a vendor checks “S Corporation” but provides a nine-digit number that starts with the format typical of an SSN, ask them to verify. EINs are formatted as XX-XXXXXXX; SSNs as XXX-XX-XXXX.
Step 2: Verify the TIN with the IRS Before You File
You don’t have to guess whether a TIN is valid. The IRS offers the TIN Matching program through its e-Services portal, available to any payer who is authorized to file 1099 forms. You can submit up to 25 TIN/name combinations interactively or batch-upload up to 100,000 records. The system returns one of several codes: match, no match, invalid TIN format, or name/TIN combination not found.
Run this check before you process the first payment over $600, not in January when you’re scrambling to file 1099s. A no-match result in November gives you time to contact the vendor and get a corrected form. A no-match result on January 28 means you’re either filing with bad data or delaying your entire 1099 run.
One practical note: the TIN Matching program only confirms that a name and number match IRS records as of a given date. It does not confirm that the vendor is in good standing, that their entity is still active, or that they haven’t had their EIN revoked. It is a necessary check, not a complete one.
Step 3: Confirm the Federal Tax Classification
Box 3 on the W-9 determines whether you even need to file a 1099 for that vendor. Corporations—both C and S—are generally exempt from 1099-NEC reporting for services. If a vendor checks “C Corporation,” you typically don’t need to send them a 1099-NEC, even if you paid them $50,000 last year. The exception: attorneys and medical providers get 1099s regardless of corporate status.
This matters because misclassification cuts both ways. If you skip a 1099 for a vendor who actually checked the wrong box and is really a sole proprietor, you have a reporting failure. Conversely, if you file a 1099 for a C-corp vendor, you haven’t done serious damage—but you’ve created unnecessary paperwork and potentially confused the vendor’s accountant.
When the classification doesn’t match what you know about the vendor, ask. A freelance graphic designer who checked “C Corporation” because they thought it sounded more professional has made an error on a federal tax form. That’s their problem to correct, not yours to accept.
Step 4: Check the Exemption Codes
Box 4 has two fields: one for exempt payee codes (1–13) and one for FATCA exemption codes. Most domestic vendors you work with will leave these blank. If a vendor fills in an exemption code, verify it against the W-9 instructions. For example, Code 5 covers corporations, and Code 1 covers U.S. tax-exempt organizations under section 501(a). If a vendor claims an exemption code that doesn’t match their stated classification in Box 3, that’s a contradiction worth flagging.
Foreign vendors are a separate category entirely. If a vendor tells you they’re not a U.S. person, they should be completing a W-8 form series (W-8BEN for individuals, W-8BEN-E for entities), not a W-9. Accepting a W-9 from a foreign vendor doesn’t protect you—it just creates a paper trail showing you didn’t apply the right process.
Step 5: Confirm the Signature and Date Are Current
An unsigned W-9 has no legal weight. The certification in Part II requires an actual signature—electronic signatures are acceptable under IRS guidelines, but the form must be signed. Check the date as well. There’s no official IRS expiration period for a W-9, but most tax professionals recommend collecting a new one every three years or whenever you have reason to believe the vendor’s information has changed: they’ve restructured, changed their name, moved to a new state, or been acquired.
If you’re onboarding a vendor who hands you a W-9 dated 2018 with a name that doesn’t match their current invoice header, ask for a new one. It takes them five minutes and saves you a B-Notice.
Step 6: Store It Securely and Track the Version
A W-9 contains either an SSN or an EIN—both are sensitive. Store completed forms in encrypted digital storage or a locked physical file, not in an email folder or a shared drive anyone in the company can browse. The IRS guidance on Form W-9 doesn’t specify a retention period, but four years is the standard recommendation to align with the statute of limitations on information return penalties.
Keep a version log. If a vendor resubmits a corrected W-9 in October, you need to know which version was in effect during January through September and which takes effect going forward. A simple spreadsheet column showing “W-9 effective date” next to each vendor record handles this cleanly.
Common Mistakes to Avoid
Accepting a W-9 at face value without running TIN matching is the single most common error—and the most preventable. Close behind it: filing a 1099 under the business name when the vendor is a sole proprietor whose SSN is tied to their personal name, which guarantees a mismatch. Teams also frequently skip re-collecting W-9s after a vendor restructures or changes ownership, then file a 1099 with outdated information. And a surprising number of businesses accept blank exemption codes from vendors who are clearly not exempt, simply because no one on the AP team knows what the codes mean. Build a one-page internal reference sheet, run TIN matching before first payment, and treat a W-9 with missing or contradictory fields as incomplete rather than close enough.
