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 a chemical vapour deposition (CVD) SnO₂ thin film under gases (air, pollutant gases CO, CH₄ or C₂H₅OH). Electrical measurements have shown that the platinum membrane thickness has to be limited to avoid a short-circuit of the SnO₂. For a small platinum layer thickness (10 nm) the conductance of SnO₂ film decreases with increasing platinum thickness. This effect is attributed to additional oxygen species on SnO₂, resulting from an enhanced oxygen dissociation by Pt. Concerning the influence on the selectivity, the measurement of the catalytic activity of Pt has been investigated. Above 500 °C, this metal is very efficient for the total oxidation of CO and C₂H₅OH, but CH₄ does not react. We tried to correlate this catalytic activity to the sensing detection properties of the SnO₂+Pt structure. Firstly, the membrane strongly decreases the sensor response under alcohol, which is true to catalytic tests. In CO case, on one hand, at high temperature (500 °C), although Pt is very efficient in regards of CO oxidation, it does not strongly decrease the sensor response. A mechanism is proposed, based on oxygen exchange between Pt particles and SnO₂ film during CO oxidation. On the other hand, at temperature lower than 150 °C, Pt membrane induces a strong increase of the conductance under CO. We have shown that this electrical phenomenon only results from Pt particles not from SnO₂ thin film. Finally, the CH₄ response is slightly decreased which is not true to catalytic properties of Pt.