Density functional theory is used to characterize reactivity in systems confined between alloy surfaces separated by a gap from three to 10 Å. It is found that the proximity of a second surface alters the geometric and electronic properties of the first one, and the changes are related to the nature of the interacting surfaces. These phenomena are explored by analysis of the dissociation of molecular oxygen and that of water in the confined systems. The results suggest that such confinement effects may be further designed for specific applications by tuning the alloy composition.