The surface compositions of a series of platinum-rhodium bimetallic catalysts supported on gamma-alumina were determined in the presence of chemisorbed hydrogen by H-1 NMR spectroscopy. The surface compositions of hydrogen-covered Pt-Rh bimetallic catalysts were found to be slightly enriched in Rh, significantly different from the surface compositions of adsorbate-free Pt-Ph surfaces, which are enriched in Pt. Further, based on selective excitation NMR experiments it was found that bimetallic particles of fairly uniform compositions were formed. Atomistic simulations of hydrogen-covered Pt-Rh bimetallic catalysts were done using a method that involved coordination-dependent bond energies. The simulations indicated that the heat of adsorption of hydrogen on rhodium is about 13 kJ/mol higher than that on platinum. Finally, a general, qualitative method for predicting the influence of adsorbates on the surface segregation behavior of bimetallic systems is described on the basis of knowledge of the surface energies at various sites and the heats of adsorption.