Abstract: Gas sensor consisting of a housing which is divided into two compartments 3 and 4 by means of a fluid filter 2, e.g. a bed of activated charcoal. Compartment 3 has an inlet channel 5 and contains a first gas-sensitive semiconductor 6, the measuring device. Compartment 4 has an outlet channel 7 and contains a second gas sensitive semiconductor 8, being used as a reference device. The value of the electric resistances of devices 6 and 8 will increase or decrease when the compartments 3 and 4 contain either an oxidizing or a reducing gas. The sensor is used in a gas detection system in which an electronic circuit which follows determines the ratio between said resistances and the ratio of the reference device and a reference resistance. These ratios are subsequently compared to certain limiting values, and when these values are exceeded, a detection signal will be produced.