It is shown that the principle of functioning of the hydrocarbon sensor recently described is not based on a new sensing effect but is the same as that of a solid state sodium concentration cell acting as an oxygen gas sensor in the presence of oxidizable gases. The particularities in the behaviour of the sensor are due to the electronic conduction properties of the solid electrolyte and the thermodynamic stability of the zeolite. (C) 2007 Elsevier Ltd. All rights reserved.