Pd-gate MOS sensors were fabricated on p-type silicon wafers. The gate films were 25 and 40 Å thick with an oxide thickness of 100 Å. Contacts were made to allow measurement of the MOS capacitance and of the impedance across the gate film. Voltage shifts in the MOS C-V curves and shifts in the Pd film impedance were measured as functions of 1) the concentration of CO and H2; 2) time as the gas ambient was varied. The devices showed sensitivity to H2at room temperature and to CO and H2at elevated temperatures. When the 25-Å device was exposed to 300 ppm H2in air at room temperature, the C-V curve shifted by -430 mV and the impedance decreased by 20 ω or 5 percent. When the 25-Å device was exposed to 5000 ppm CO in air at 150°C, the C-V curve shifted - 200 mV and the impedance decreased by 140 ω (10 percent). When exposed to 0.1-percent H2in argon, the resistance of the 40-Å device increased by about 2 percent. When measured as a function of time, the changes in MOS capacitance tend to track the changes in impedance. An effect similar to hydrogen-induced drift (HID) was observed for CO at elevated temperature.