An RFID system includes a radio frequency identification (RFID) reader and at least one passive RFID sensor tag. A backscattered radio frequency signal from the passive RFID tag is modulated with an oscillation frequency which is dependent on a value of a quantity sensed by a sensing element. The RFID reader converts converting the oscillation frequency of the backscattered signal into an actual value of the sensed quantity based on predetermined sensor configuration information of the passive RFID sensor tag. The sensor configuration information includes information on a sensor element or sensor elements available in the passive RFID sensor tag, and particularly information how to convert the oscillation frequency of the backscattered signal into actual values of the quantity sensed by the respective sensing element.