Microstructural changes in the surface crystallinity of supported platinum with time in aging were examined by means of microfocused X-ray diffraction technique. Two typical commercially available platinum catalysts in pellet form, 0.5% Pt-Al 2O 3, were served for automotive exhaust oxidation under nonleaded engine dynamometer driving condition. It was revealed that the platinum-dispersed alumina layer on the support, extending approximately 100–200 μm deep from the surface, possessed a disordered crystalline arrangement distinguishable from the usual form of platinum and from the internal alumina matrix. The initial adherent form of platinum with rather well crystalline character exhibited a higher activity of CO and HC (hydrocarbons) oxidation, than the one with amorphous form of platinum. It has been shown that deactivation of catalyst under nonleaded condition is closely associated with alteration in crystalline orientation of this characteristic thin zone of the catalyst surface.