Fine dispersions of metal aggregates such as Pt, Pd, or Cu in metal oxides are well known to improve the electrical sensitivity and selectivity of solid-state gas sensors put in contact with reducing gases (CO and H2S, for instance) of solid-state gas sensors. The improvement originates in the catalytic role played by the aggregates in regard to gas oxidation/reduction. The details of the solid-gas interactions are nevertheless not fully understood. To further investigate the role of Pr aggregates in the sensing mechanisms results of in situ X-ray absorption and Anger electron spectroscopy from Pt/SnO2 films have been combined. Films with a Pt concentration in the range 0.6-6 atom % have been prepared using a modified chemical vapor codeposition method. Results show that the sensing mechanism in mainly driven by the CO oxidation catalyzed by the aggregates: first a spillover effect occurs between CO, chemisorbsd oxygen, and PtOy aggregates, and in a second step, after a period of time depending on temperature, the aggregates are reduced/oxidized provided that they are small enough. The correlations between Pt particle behavior and film electric properties are discussed.