The morphology of 25 Å porous Pt films deposited on thermally grown SiO2 at room temperature and 250 °C is presented. The impedance of the films was measured over the frequency range 100 Hz to 1 MHz. At 1 kHz, the impedance was measured in electric fields up to 250 V/cm, over the temperature range −192–140 °C. The ac impedance of the 25 Å Pt film deposited at room temperature decreased by 25% in 21 days at 150 °C in room air but the ac impedance of the Pt film deposited at 250 °C did not show any change. The stable 25 Å film was used as a gate in a metal–oxide-semiconductor (MOS) sensor, which was exposed to 800 ppm H2/air at 100 °C and 1% CO/air at 150 °C. Measurements were made of the change in the impedance of the gate film and the change in the MOS capacitance.