Using first principles techniques we have examined the relative physisorption/chemisorption energetics of neutral molecular water and oxygen at the most thermodynamically stable surface (110) of tin dioxide. We find that water binds more strongly to the perfect surface at 5 co- ordinate tin sites than oxygen. However, binding of both water and oxygen at bridging oxygen vacancies in the defective surface is comparable. In the context of gas-sensing behaviour at moderate temperatures (∼300K), we propose that the Mars and van Krevelen [1] re-oxidation reaction will slow when the partial pressure of water is high, since the number of favourable adsorption sites will effectively decrease. In addition, one would expect that the surface conductivity will increase, since the re-oxidation reaction will be hindered.