New aspects of the influence of water vapour in the interaction of SnO 2 gas sensors with different reducing gases are evidenced by an original experimental method. This consists in measuring the conductance transients during and after rapid transitions from dry into humid air containing identical concentrations of methane and CO, respectively. The results indicate that while methane and water compete in reacting with one type of oxygen ions on the surface, CO reacts mainly with another type of surface oxygen, almost insensitive to water.