In Pt-doped SnO2 thin films prepared by using a submicroscopic aerosol pyrolysis method where the metal elements are codeposited, the best CO sensitivity is obtained from low concentration Pt precursor solutions. Transmission electron microscopy (TEM) observations of SnO2 thin films doped by small amounts of platinum have been performed to analyze in detail the structural properties as a function of Pt doping concentration. Strong evidence of porous structure has been found in the TEM images and an assessment of the porous volume fraction could be done. Although this approach clearly underestimates the assessed porous fraction, results for undoped SnO2 films nevertheless indicate that the film density is close to half of the bulk value. It increases with the film doping concentration, [Pt]/([Sn] + [Pt] + [O]), as films become thinner. For doping concentrations higher than 2%, no significant number of pores could be detected and the thickness reaches a plateau likely corresponding to the maximum density that can be obtained using our synthesis conditions. The usually observed maximum in the film sensitivity at solution concentrations close to 0.6% film doping, i.e., 3% in terms of solution concentration, coincides with a maximum in the product of the porous volume and the density of small Pt aggregates. Additionally, high-resolution electron microscopy observations of selected film samples have been performed to examine grain arrangements and microstructural effects of Pt and SnO2 nanocrystallites.