The lattice constant of Pt0.7Co0.3 alloy particles prepared on a carbon black support between 200 and 1000 degrees C by an impregnation method with ethanolic solutions of Pt (NH3)(2)(NO2)(2) and Co(NO3)(3) decreased with an increase in the preparation temperature up to 600 degrees C and then attained a constant value; a = 0.3827 nm above 600 degrees C. This value shows the formation of the well homogeneous Pt0.7Co0.3 particles. The catalytic activity per real surface area of the Pt0.7Co0.3/C catalysts for the oxidation of methanol in 0.5 M H2SO4 increased with an increase in the preparation temperature up to 600 C and then slightly decreased beyond the temperature. At a preparation temperature higher than 800 degrees C, "grooving" of the carbon black support by the Pt0.7Co0.3 alloy particles was observed. This grooving phenomenon was confirmed with model catalysts using a highly oriented pyrolytic graphite (HOPG) support. On the HOPG surface, Pt0.7Co0.3 nanoparticles grooving of the HOPG support from the edges to the terraces was influenced by the hexagonal carbon atom arrangement of the HOPG; the mutual direction of the grooves; i.e., nanochannels, was parallel or at 60 degrees although random grooving was also observed. (c) 2005 The Electrochemical Society. All rights reserved.