RFID (Radio-Frequency Identification) tags are small, inexpensive microchips capable of transmitting unique identifiers wirelessly over a short distance. Thanks to their utility in automating supply-chain logistics, RFID tags promise eventually to supplant the optical barcode as a means of identifying goods. We propose the concept of a yoking-proof, namely a proof that a pair of RFID tags has been scanned simultaneously. Our particular aim is to permit tags to generate a proof that is verifiable off-line by a trusted entity, even when readers are potentially untrusted. We suggest that such proofs are a useful tool for maintaining integrity in supply chains, particularly as RFID data will commonly flow across multiple, loosely affiliated organizations.