The prewetting transition in the silica-water-2,5-dimethylpyridine system has been studied through the adsorption isotherms. Steep wave shapes for these isotherms lead to the conclusion of alternate solid and liquid surface demixing very close to bulk coexistence and wetting temperature. Liquid demixing has the generic aspects of prewetting. An empirical model for demixings, surface phase diagrams in several representations, and critical end points are discussed, as well as the phenomena universality. (C) 1997 American Institute of Physics. [S0021-9606(97)51041-5].