This paper describes an investigation of the possibility of measuring small amounts of carbon dioxide in air by the absorption of infra-red radiation. It is shown that the method is simple, trustworthy and accurate, and is very sensitive for small amounts of carbon dioxide, of the order of that present in ordinary air. The principle is applied in a form superior to any hitherto used, in that the whole of the transmitted radiation is measured instead of merely the radiation at the maximum of a particular absorption band. This makes a spectrometer unnecessary, eliminates the disturbing effect of variation of temperature, and allows much less sensitive detecting apparatus to be used. The only preliminary treatment necessary is the removal of water vapour from the air under examination.