A number of potentiometric gas sensing electrodes using a hydrogen or other ionsensing electrode and a gas permeable membrane have been studied. A theoreticalmodel has been developed describing the time response, electrical potentialbehaviour and limit of detection as a function of membrane properties, geometry,and internal electrolyte composition. Electrodes responding to carbondioxide, ammonia, amines, sulphur dioxide, nitrogen dioxide, hydrogensulphide, hydrogen cyanide, hydrogen fluoride, acetic acid, and chlorine havebeen constructed and their response characteristics are found to be in goodagreement with the theoretical predictions.