The selectivity of reactions on active patches over those on a partially active carrier surface is investigated for patch-covered spheres. The effects of relevant parameters including patch and carrier reactivities, patch coverage, patch size, and patch separation state are studied. The selectivity is found to increase with increasing relative dominance of the patch reactivity over the carrier reactivity and patch coverage, as expected. Interestingly, it also increases with decreasing patch size and increasing patch separations. Decrease in patch size creates more patch-carrier interfaces and thus enhances the competition effectiveness of the patches against the carrier surface. With respect to the patch separation state, more complete utilization of the patches is achieved for well-separated patches and thus enhances the selectivity toward the patches. (c) 2007 American Institute of Chemical Engineers.