The construction and performance of gas-sensing membrane probes are described and discussed. Experimentally, probes to measure ammonia, sulphur dioxide and nitrogen oxide in aqueous solution have been studied. In particular, the effects of temperature and osmotic pressure were examined, the response speeds of the probes were determined and their use in continuous analysis was investigated. Selectivity is discussed. Consideration is given to the development of new methods of analysis and current applications of the probes are mentioned.