Abstract: A motor vehicle theft deterrent device comprises a timer (2) operable by an existing alarm system (1), a relay (3) actuable in response to the timer (2), and a valve (14) operable, via the relay (3), to release and direct, through one or more concealed nozzles (4), a spray of a pressurized, non-toxic and non-corrosive irritant towards a would-be thief. The circuitry (12) and a pressurized container (9) may be accommodated in a tamper-proof metal housing (7) mounted upon the firewall of the vehicle, and a secondary circuit (5) may be activated so as to cause a siren or loudspeaker (6) to emit a high-pitched sound.