This letter presents a printed UHF RFID sensor solution that indicates if a passive RFID tag has been exposed to a certain degree of moisture. The printed sensor operates as a write-once-read-many (WORM) resistive memory device as it permanently changes its resistance from about 10 to 10 after exposure to moisture or water. A printed coupling loop with an embedded WORM sensor is horizontally placed just above the surface of an ordinary UHF RFID tag. Electromagnetic coupling is used to modulate the properties of the tag antenna by changing its input impedance and introducing ohmic losses in proportion to the embedded sensor values. The passive RFID tag can change state from readable to unreadable when the WORM bit is set, i.e., is put in a low-resistance state. The proposed concept verifies that commercial RFID tags can be used as sensor tags by simply adding an electromagnetically coupled sensor as a sticker or by similar means, without the need for ohmic contacts between the sensor and the original RFID tag. Â© 2006 IEEE.