The characteristics of Sc2O3/AlGaN/GaN metal-oxide semiconductor (MOS) diodes and Pt/ZnO Schottky diodes as detectors of C2H4 are reported. At 25degreesC, a change in forward current of similar to40 muA at a bias of 2.5 V was obtained in response to a change in ambient from pure N-2 to 10% C2H4/90% N2. The current changes are almost linearly proportional to the testing temperature and reach around 400 muA at 400degreesC. The mechanism of the change in forward gate current appears to be formation of a dipole layer at the oxide/AlGaN interface that screens some of the piezo-induced channel charge at the AlGaN/GaN interface. The ZnO diodes show no detectable change in current when exposed to ethylene at 25degreesC but exhibit large changes (up to 10 mA) at higher temperatures. In these diodes the detection mechanism appears to also involve introduction of hydrogen donors into the near-surface region of the ZnO, increasing the effective doping level under the rectifying contact. (C) 2004 The Electrochemical Society.