Merchant Identification Numbers (MIDs) and their role in the Payment Industry

During my first two weeks at Finix Payments, one term that kept appearing was the term MIDs.

What is a MID?

A Merchant Identification Number (MID) is a unique number or code provided to merchants by their payment processor. The code is often abbreviated as MID and transmitted along with the information of the cardholder to complete a transaction. The MID identifies a merchant when transacting with their payment processor and other involved parties.

Does everyone get a Merchant Identification Number (MID)?

Not everyone gets a MID. For example, payment services like Square and Stripe don’t use MIDs. They take away that complexity at a cost of higher credit card transaction fees.

As a consumer, you can often see MIDs on receipts. For example, at a Korean restaurant in Oakland, the MID was printed at the bottom of the receipt. You won’t see the full number but usually the last 4 digits are present.

MIDs and Credit Card Fraud

Since MIDs are one of the key information processors use to identify merchants, it’s important to keep it secure. Someone who is attempting fraud could potentially use your MID and other data points to process fraudulent transactions.

If you have too many fraudulent transactions under your MID, your processor will shut down your account. Consumers who have experienced fraud will likely start a chargeback process through their bank. Most processors will close your account if you have too many chargebacks.


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