Density functional calculations for the dissociation of molecular oxygen on a Ag-10 cluster model (seven surface and three subsurface atoms) of the [111] surface of silver are presented. The calculations show that the reaction pathway for the dissociation of molecular oxygen depends on the amount of cesium present on the surface. For the unpromoted surface, dissociation should have a low rate as the process is activated. As a single Cs atom is added to the cluster, O-2 should readily dissociate since the process is kinetically and thermochemically favored. However, as a second Cs atom is added, the dissociation of O-2 is quenched as the process is highly activated.