Water vapour adsorption was investigated on SnO2 surfaces prepared by oxidizing metallic tin with nitric acid. The adsorption isotherms reveal a discontinuity, i.e., a sharp increase in the amount of adsorbed water in the range of relative pressure 0.02–0.03; this is the second example of metal oxide + water systems exhibiting a discontinuity in the adsorption isotherm after the system ZnO + H2O. The height of the discontinuity decreased gradually by successive evacuation of the sample at higher temperatures in the order of 313, 573 and 773 K (series 1) or of 1073, 573 and 773 K (series 2), and finally the discontinuity disappeared. Plots of isosteric heat of physisorption of water show a plateau in the same range of coverage as that corresponding to the discontinuity. The plateau also decreased by successive heating of the sample, in parallel with the decay of the discontiniuity. The adsorption data show a good fit to the Hill–de Boer equation in the range of the discontinuity. The structure of the adsorbed water layer on SnO2 is discussed, by evaluating the ratio of the number of physisorbed molecules in the monolayer to that of underlying surface hydroxyls.