The structure and hydrogen-induced conductometric response of thin nanoparticulate platinum films has been investigated. Platinum films with a nominal thickness of 35 Angstrom were deposited on silicon oxide and stabilized by thermal treatment in air at 673 K, resulting in an average platinum particle diameter of approximately 30 nm. These platinum films exhibited a positive temperature coefficient of resistance and resistance values intermediate between ultrathin and thick films. Exposure to ppm levels of hydrogen in the presence of 5% oxygen with nitrogen as the carrier gas caused decreases in electrical resistance. In the temperature range between 300 and 570 K, the relative response became more pronounced with increasing temperature, indicating that the response mechanism for hydrogen sensing is an activated process. In the temperature range of 370-470 K, the hydrogen concentration dependence of resistance changes can be divided into two nearly linear regimes. From 10 to 200 ppm of hydrogen, the response (Delta R/R-0/C-H2) is 0.36/1000 ppm H-2, while above 200 ppm, the response is 0.01/1000 ppm. The decreased response at higher hydrogen concentrations can be attributed to saturation of the active sites for hydrogen oxidation. The response is significantly decreased in the absence of oxygen, suggesting that the surface-catalyzed hydrogen-oxygen reaction plays an important role in the sensing mechanism.