Abstract: A method of detecting for the presence of a combustible infrared absorbing gas in air, particularly in hazardous locations such as on gas and oil rigs. A source of switched infrared radiation is beamed onto a detector within a chamber and an output signal is derived as a measure of radiation incident on the detector. A reduction in the level of the output signal indicates absorption of radiation within the chamber and, thus, indication of the presence of infrared absorbing gas within the chamber. The output signal is processed in a manner which makes provision for the existence of changing levels of background radiation which would otherwise result in a false indication of the presence or absence of combustible gas.