A CMOS gas sensor comprises a membrane (13) extending over an opening (12) of a silicon substrate (1). A patch (2) of sensing material is arranged on the membrane (13) and in contact with electrodes (3) of platinum. A heater (5) of tungsten is located in or on the membrane (13) at the location of the patch (2) of metal-oxide sensing material. Combining platinum electrodes (3) with a tungsten heater (5) on top of a CMOS structure provides a gas sensor of high reliability and stability.