Abstract: A gas analyser of the infra-red radiation absorption type, particularly for measuring the amounts of CO.sub.2 and CO in vehicle exhaust gases, comprises a pair of infra-red radiation sources arranged to direct their radiation along a measurement path and a reference path respectively, the radiation entering each path being periodically interrupted by a rotating commutator. Two measurement chambers for receiving the gas to be analysed and respective detection chambers for CO.sub.2 and CO are alternately disposed along the measurement path, while two reference chambers and two more such detection chambers are alternately disposed along the reference path, corresponding detection chambers being connected to respective differential pressure sensors of the variable capacitance type. The analyser includes a calibration cell containing predetermined concentrations of CO.sub.2 and CO. The cell is selectively interposable in the measurement path, and is connected to a flexible-wall chamber.