The admittance of 25 Å palladium-gate MOS capacitors with 125 Å of silicon dioxide has been studied as a function of hydrogen gas concentration and carbon monoxide gas concentration. The ultra-thin gate is a porous film, consisting of an array of partially connected islands. This MOS capacitor is sensitive to hydrogen (3 ppm to 1%) at room and elevated temperatures and to carbon monoxide (100 ppm to 10 000 ppm) at 150 °C. Using a bias scan conductance method at a fixed frequency, the interface state density ( N it) is determined at the surface potential corresponding to the peak in the conductance curve. It is shown that at the corresponding location in the bandgap, N it increases with increasing concentrations of hydrogen, while N it decreases with increasing concentrations of carbon monoxide.