Surface chemical composition and topography of Pt-SnO2 thin films grown by radiofrequency (rf) reactive sputtering on two different Al2O3 substrates (rough and mechanically polished) were investigated by x-ray photoemission spectromicroscopy (XPSM) and atomic force microscopy (AFM). XPSM measurements showed, for both substrates, a homogeneous chemical composition of the Pt-SnO2 films. The only difference was the observation of different charging in different areas of the film grown on rough alumina substrates, due, presumably, to a non-continuous Pt-SnO2 film. AFM showed large topographical variations (several hundred nanometres) for the Pt-SnO2 film grown on a rough alumina substrate, due to structures already present on the substrate. The estimated roughness of the sensor was 20% larger for the Pt-SnO2 film grown on a rough alumina substrate. The response to carbon monoxide was 30% higher for the sensor grown on rough alumina than that on polished alumina, reflecting the larger exposed sensor area.