The paper presents the results of calorimetric measurements and surface excess adsorption isotherms for n-butanol adsorption from n-hexane on a series of controlled porosity glasses characterized by different mean pore diameters. It is demonstrated that, in the region of very low alcohol concentration in solution, the heat of adsorption exhibits sharp maximum, independently of the pore diameter of adsorbent. This rather puzzling result is explained by the heterogeneity of the surface and the effects of molecular association of alcohol molecules in the solution. A simple theoretical model that supports our predictions is presented.