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The hydrolysis of H 2 PtCl 6 , RuCl 3 , OsCl 3 , and H 2 IrCl 6 under basic conditions in the presence of a watersoluble betaine stabilizer affords aqueous colloidal solutions of the mixed metal oxide PtRuOsIrO x displaying a particle size of 1.3-1.6 nm. The ratio of the four metals can be adjusted by choosing the appropriate relative amounts of metal salts. Characterization was accomplished by TEM, XPS, and XRD. Immobilization of the reduced form on high surface conducting carbon leads to materials which are excellent electrocatalysts showing unusually high resistance to CO poisoning.