A possible way to improve the selectivity of tin dioxide sensors is to deposit a membrane with specific properties above the sensing element. The purpose of this paper is to investigate the effect of a platinum membrane on the sensing properties of thin (chemical vapour deposition) and thick (screen-printing technology) SnO₂ films under gases (air, diluted pollutant gases CO, CH₄ or C₂H₅OH). The measurement of the catalytic activity of Pt has been investigated. At above 500 °C, this metal is very efficient for the total oxidation of CO and C₂H₅OH, but CH₄ does not react. As a consequence, it should be possible to develop a selective sensor to CH₄ in presence of C₂H₅OH and CO as interfering gases. Firstly, we tried to correlate this catalytic activity to the sensing detection properties of the thin SnO₂ film+Pt structure. The membrane strongly decreases the sensor response under alcohol, and does not affect the CH₄ one, which is in agreement with catalytic tests. In CO's case, at high temperature (500 °C), although Pt is very efficient with regard to CO oxidation, it does not strongly decrease the sensor response. To explain this result, a mechanism based on the contact between Pt particles and thin SnO₂ film is proposed. Hence, in order to decrease the CO's response, Pt membrane has to be insulated from SnO₂ film. A new structure, thick SnO₂ film+thick insulator coating (SiO₂)+Pt, has been tested. A good correlation between catalytic tests and the effects on the sensor response is obtained. However, contrary to thin film device, a good correlation is also obtained in the thick SnO₂ film+Pt structure when a contact between Pt and SnO₂ is materialised. In thick-film structures, no insulator coating seems to be necessary.