Abstract: A gas detector 10 comprising a first radiation source 12 for emitting radiation at a first frequency, a second radiation source 14 for emitting radiation at a second frequency, a radiation detector 18 adapted simultaneously to detect temporally overlapping radiation from both the first and second radiation sources which in use pass through a sample region 16 located between the first and second radiation sources 12, 14 and the detector 18, and further, comprising a processor 20 enabling comparison of the radiation detected by the detector 18 from the first and second radiation sources 12, 14 thereby to determine the level of a pre-determined gas in the sample region.