SnO2 thin films have been grown by radio frequency reactive sputtering method in order to be used as a gas sensor. Subsequently Pt has been added to the SnO2 films to increase the sensitivity to carbon monoxide gas. Two different substrates have been used : rough and mechanically polished alumina. The morphology of the film has been characterized by atomic force microscopy, whereas the chemical composition was analyzed by x-ray photoemission spectromicroscopy. Pt:SnO2 clusters with widths of 500-600 nm were observed on the rough alumina, whereas on the mechanically polished alumina, the Pt:SnO2 film was smoother. X-ray photoemission spectromicroscopy measurements show for Pt:SnO2 films on rough alumina substrates different charging in different areas of the sample. The response curve to carbon monoxide is 30% higher for rough alumina.