Hydrogen chemisorption measurements of highly dispersed Pt, Rh, and Ir catalysts yielded H/M values exceeding unity. These results cannot be used straightforwardly to determine the average metal particle size, because the H/M stoichiometry on the surface is unknown. EXAFS measurements were performed to determine the metal particle size and thereby, to calibrate the hydrogen chemisorption results. The high hydrogen chemisorption values can be explained best by assuming H/M surface stoichiometries exceeding unity. The adsorption stoichiometry differs among Pt, Rh, and Ir in the order H/Pt <H/Rh <H/Ir, analogous to the order in stability of the corresponding metal polyhydride complexes.