It has recently been established that the electrical properties of tin oxide are greatly modified after a sulfur dioxide treatment at 500°C. According to the type of gas present in the atmosphere, its electrical conductance curves as a function of temperature exibit maxima at different temperatures. Such a maximum may be understood in terms of potential barrier at the SnO₂ metal junction and in terms of a chemical reaction which induces an increase of the sample's conductance at low temperature. The surface states of SnO₂ , and consequently the nature of the water vapour present on the surface, are greatly affected by the gaseous treatment.