A method of operating a selective combustible sensor having a pump cell and a sensor cell includes the steps of immersing the sensor in a gaseous atmosphere having at least two combustible gases and oxygen gas and allowing the gaseous atmosphere to enter a region between the pump cell and the sensor cell, applying a pumping current to the pump cell, sensing an e.m.f. (V.sub.s), across the sensor cell, varying at least either the V.sub.s or temperature to make the sensor insensitive to one of the combustible gases, and detecting the other combustible gases present in the gaseous atmosphere.