The present theories of surface premelting are based on the assumed mechanism whereby a liquid layer is formed because the free energy of the surface is thus minimized. It is shown experimentally here that this mechanism is not possible in the case of ice, for which surface melting below 0 °C is, however, well documented. A mechanism of surface melting, unique for ice, is then proposed. It is based on a concept according to which the surface phase equilibrium temperature differs from that of the bulk material. The new theory predicts the onset temperature of premelting on ice in quantitative agreement with recent experimental data. It also explains regelation of ice and explains why water reaches a density maximum above its freezing point.