Abstract: Apparatus for detecting a target gas in a monitored space includes two laser diodes driven by drive circuits at electrical frequencies f and f? that are not harmonically related. The lasers operate at mean wavelengths ?1 and ?2 respectively close to two different absorption lines of the target gas and are scanned over wavelength ranges ??1 and ??2 respectively. The outputs from the lasers are collimated by an optical element and delivered to a receiver element after passing through the space. The receiver element focuses the radiation from both lasers onto a detector where the optical signals are combined into a single electrical signal with principal frequency components f and f?. A quantity of target gas Q1 is calculated from the amplitude of frequency component f1 for measurements made around wavelength ?1 and a quantity of target gas Q2 is calculated from the amplitude of frequency component f2 for measurements made around wavelength ?2.