Abstract: An internal combustion engine spark ignition system is provided which ensures that sparks are inhibited if the engine temperature exceeds a maximum safe level. The ignition circuit includes an input transistor which is switched on and off by an engine shaft transducer. The input transistor controls charging and discharging of a capacitor the voltage on which determines whether switching of the input transistor causes a spark to be produced via an output amplifier and ignition coil. A temperature sensing element determines both the rate at which the capacitor can charge and the final voltage to which it can be charged.