A gas sensor of the type that detects the presence of a specific gas by monitoring the absorption of optical radiation transmitted through a chamber containing a sample of gas under test comprises an optical source for emitting radiation therefrom and a detector sensitive to radiation emitted from the source at opposing ends of a circumferential chamber, having optically reflective surfaces, extending around the periphery of a sensor housing. The optical pathway between the source and detector may include a radial portion, a circumferential portion and an axial portion to allow a compact optical path. The gas sensor includes, within a single housing, electronic circuitry for conditioning the electrical output of the detector to provide an output that is a function of at least one selected gas concentration and which is automatically compensated for at least one of temperature, pressure, humidity, and range normalization.