Abstract: A sensor element responding to gases and/or vapors is heated in at least two temperature cycles continuously from a starting temperature value to an upper temperature value in accordance with a predetermined pattern which is optimized for selective gases. The temperature is lowered to the starting temperature value in accordance with the same or a different pattern. During the temperature cycles a signal which depends on the composition of the gas or vapor atmosphere delivered by the sensor element is compared in an electronic evaluation circuit to stored values which are characteristic for the presence of selective components in the gas and/or vapor.