The electrocatalytic activity of bimetallic Pt0.5Ru0.5N(oct)(4)Cl colloids supported on Vulcan XC72 toward the oxidation of CO and CO/H-2 mixtures was investigated by using a newly developed method to characterize supported catalysts in a rotating disk electrode configuration. Prior to the electrochemical measurements, the surfactant was removed by reactive annealing. Parallel atomic force microscopy (AFM) measurements on a planar model system [Pt0.5Ru0.5N(oct)(4)Cl on highly oriented pyrolitic graphite] indicate minor sintering in good agreement with high-resolution transmission electron microscopy (HRTEM) measurements on the Vulcan-supported catalyst which yield a dispersion of similar to 43%. At 60 degrees C, the electrochemical activity of the PtRu colloid catalyst toward the oxidation of 2% CO in H-2 is equal to or slightly higher than that of a commercial PtRu catalyst (E-TEK). Comparison of the CO tolerance of the PtRu-colloid/Vulcan with that of proton exchange membrane (PEM) fuel cells was facilitated on the basis of mass-specific current densities, providing a scale-up method for predicting the performance of catalysts in a PEM fuel cell.