Abstract: A chemically activated rotary actuator mechanism is described which reacts to exposure to hydrocarbons such as gasoline and oil to cause a rotor to forcibly rotate from a rest or static position. Spring members load the rotor with compressive forces which act through the rotational axis of the rotor to cause the mechanism to be inherently unstable in the static position. The rotor is rotatably mounted in a body member and constrained from rotation with respect to the body member by means of shear pins which degrade when exposed to hydrocarbons. Rotation of the rotor shaft is utilized in generating an alarm.