Preferential adsorption of gases onto metal surfaces through defect, edge, and corner sites has been well documented. What has nut been generally appreciated, however, is that the addition of a second metal can alter adsorption kinetics by preferentially occupying these relatively lower coordination surface sites. We propose a new model for H-2 adsorption onto supported Ru-Ag catalysts that explicitly accounts for preferential adsorption through surface edge and corner sites, and we show that the model can explain experimentally observed decreases in the amount of H-2 adsorbed per surface ruthenium atom upon increasing the amount of silver in the catalyst.