The adsorption of 2, 5 DMP (2, 5-dimethylpyridine) at the free surface of water (1)-2, 5 DMP (2) liquid mixtures was determined from surface tension and activity measurements. Two divergences were found: the former was observed at Tc for the critical isochore and the 2, 5 DMP-rich γ-phase which, at coexistence, completely wets the surface at Tc, whereas the latter was noticed at Tw for the water-rich β-phase. These results were compared with those found under similar conditions for the same liquid system with silica as the wall; this comparison is quite fruitful because, with silica, the preferentially wetting phase is the β-phase.