A seat occupancy sensor comprises a pressure detection device associated with a surface of a seat and a control unit for communicating with the pressure detection device. According to the invention the pressure detection device comprises a surface acoustic wave device including at least one surface acoustic wave resonator and an antenna and the control unit comprises an RF antenna for remotely communicating with said surface acoustic wave device. In a first embodiment, the surface acoustic wave is adapted for detecting the pressure inside a sealed chamber. In a second embodiment, a pressure sensitive switching device is connected to the surface acoustic wave device so as to activate the surface acoustic wave device when the pressure sensitive switching device is triggered.