The surface reactivity of metal oxide materials can be enhanced by nanoparticle decoration, which is of crucial importance in catalysis and chemical sensing applications. Here, we employ ultrasmall Pt nanoparticles for the functionalization of tin oxide (SnO2) thin film-based chemoresistive sensors integrated in complementary metal-oxide-semiconductor technology. Size-selected Pt nanoparticles with an average diameter below 2 nm were fabricated by a solvent-free gas-phase synthesis approach and deposited onto the SnO2 sensing layer surfaces, which resulted in carbon monoxide sensing properties with minimized humidity interference. The atomic-scale structure of ultrasmall Pt nanoparticles supported on SnO2 was studied by in situ transmission electron microscopy, performing heating experiments in reactive gas atmosphere relevant for sensor operation. Our results reveal the formation of Pt oxide phases and nanoparticle-support interactions, which contributes to a more detailed understanding of the structure-property relationships in the SnO2-Pt nanomaterial system.