We have used first-principles calculations to understand water adsorption on the SnO2(110) surface. Particular attention has been paid to the roles played by surface geometry and inter-adsorbate interactions. For this purpose a full comparison between adsorption on SnO2 and TiO2 surfaces is made. There are several points of contrast between adsorption mechanisms for the two materials. The most significant differences are that molecular water is only stable on the SnO2 surface when bound to a neighbouring OH group, and that the results favour dissociative adsorption on SnO2 at monolayer coverage. The 'mixed adsorption state' first suggested for TiO2 [1] is metastable on SnO2(110). For both materials there are small energy differences between alternative adsorption states at monolayer coverage.