A battery-driven micro electromechanical system (MEMS) gas sensor has been developed for household safety when using natural gas. The heart of the MEMS gas sensor is a 7.5 at % Pt−SnO 2 thin film catalyst deposited on the SnO 2 sensor layer. The catalyst enhances the sensitivity to methane, though its structure under working conditions is unclear. In this study, in situ XAFS was applied to a 7.5 at % Pt−SnO 2 catalyst layer deposited on a Si substrate, and we demonstrated that atomically dispersed Pt maintains its lattice position in SnO 2 with a small loss of surrounding lattice oxygen in the presence of 1% CH 4 and a more reducing gas of 1% H 2 at the reaction temperature (703 K), i.e., no Pt aggregation is observed. The lost oxygen is easily recovered by re-oxidation by air. This work has revealed that the atomically dispersed Pt in the SnO 2 lattice is the active structure and it is stable even under reaction conditions, which guarantees a long lifetime for the gas sensor.