The bulk-composition-dependent dissociative adsorption energy of hydrogen on CuPd alloys has been measured experimentally and modeled using density functional theory. The hydrogen adsorption energy cannot be simply defined by a single reactive site or as a composition weighted average of the pure metal components. We developed a modeling approach that uses a basis of active sites weighted by a model site probability distribution to estimate a bulk-composition-dependent adsorption energy. The approach includes segregation under reaction conditions. With this method, we can explain the composition-dependent adsorption energy of hydrogen on Cu-rich alloy surfaces. In Pd-rich alloys, a Pd-hydride phase may form, which results in deviations from trends on the metallic alloy surface.