Portable instruments are described for the continuous determination of sulphur dioxide and carbon monoxide in air. For sulphur dioxide, the gas is absorbed in an aqueous electrolyte containing an acid-base indicator. Resulting changes in the indicator colour are monitored photoelectrically and the detector current, after amplification, is used to titrate the reaction product. The titration current can be used to calculate the gas concentration at a given flow-rate, on a coulometric basis. The changing current is displayed on a chart recorder, which provides a record of concentration against time. Carbon monoxide, after conversion into carbon dioxide, is determined in a similar manner using a partially aqueous electrolyte. Ranges of 100 and 20 mg m–3 for sulphur dioxide and 250 and 50 p.p.m. by volume for carbon monoxide are provided. These ranges cover the Threshold Limit Values for the gases. The instruments, which are battery powered, can be left unattended on-site for periods of at least 8 h. They have been used successfully for occupational hygiene studies in steelworks environments.