The adsorption properties of binary alloy surfaces may significantly deviate from those of the pure component metals. The physical origin of this deviation arises in a rather complex way from the distribution of both kinds of metal species and their interaction in the surface. In contrast to most other adsorption work on binary alloy surfaces we demonstrate and discuss here a clear-cut distinction between a pure geometrical ensemble effect and an electronic ligand effect on the adsorption of gas molecules by using an ordered alloy surface, namely Cu3Pt(111). Furthermore, we show the extent to which the chemical heterogeneity of the surface playing a role depends also on the nature of the adsorbate.