A new sensitive method has been developed to determine the percentage of metal exposed (% ME) in commercial three-way bimetallic catalysts, fresh and aged. The method is based on the measurement of specific rates of two structure insensitive reactions: the hydrogenation of benzene and propene. Values of % ME were obtained for each metal at the surface of Pt-Ph and Pd-Rh catalysts with very low % ME. The % ME and the corresponding catalytic activity of the samples in the postcombustion follow the same trend as a result of aging.