A sensor for detecting CH4 in exhaust gases from gasoline engines has been constructed from two Pd\YSZ\Au cells, which are attached to each other with their Pd electrodes in the interior side of the assembly. When the sample gases consisting of CH4, O-2 and H2O were fed to the sensor element at 750 degrees C, EMF values were generated from the one cell and then compensated by pumping O-2 electrochemically from the other cell. Currents applied to the latter cell linearly increased with CH4 concentration, but were not influenced by changing O-2 concentration.