An alloy catalyst of 15 wt % Pt50Ru50/C was prepared by the method of incipient wetness impregnation and activated by hydrogen reduction at 620 K. Physical characterization of the freshly reduced catalyst indicated that bimetallic crystallites, Pt rich in the shell and Ru rich in the core, were finely dispersed in a diameter of d(PtRu) similar to 2 nm on carbon support. The reduced catalyst was subsequently modified by oxidization in air. On increasing the temperature of oxidation (T-o), atoms of Ru in the core were found segregated to the surface of bimetallic crystallites and oxidized to amorphous RuO2. Crystalline RuO2 ((RuO2)-O-c) was formed on extensive segregation at T-o > 520 K. Catalytic activity of the alloy catalyst for electro-oxidation of methanol was examined by cyclic voltammetry. Electrochemical activity of the Pt-Ru/C catalyst was found to be significantly enhanced by oxidation treatments. The enhancement was, therefore, attributed to the segregation of Ru and the formation of RucO(2). Extensive oxidation treatment at elevated temperatures of T-o > 600 K, however, caused the deactivation of the electroactivity. The deactivation should be the result of excessive oxidation of the carbon support.