A sensor element detecting propylene at high temperatures has been fabricated by attaching two Pd or Pt\YSZ\Au cells together where the Pd or Pt electrodes were located on the inside of the assembly. The sensing properties of the sensor at 750 degrees C were investigated using a flowing mixture of propylene, methane, carbon monoxide, and oxygen, the concentrations of which were in the range of 100 to 700 ppm. The sensor element using the Pd electrode had a high sensitivity to propylene, but a low selectivity to propylene over methane. The use of the Pt electrode made it possible to significantly improve the selectivity to propylene over methane, because the catalytic activities of the Pt electrode for the oxidation of methane were far lower than that of the Pd electrode and as low as that of the Au electrode. The current signal at a fixed oxygen concentration of 300 ppm was almost proportional to the propylene concentration. In addition, the current signal at a fixed propylene concentration of 350 ppm was not significantly affected by changing the oxygen concentration.