PtRu-modified An nanoparticles on a carbon support were prepared using a polyol reduction process with various Pt/Ru ratios. The prepared nanoparticles were characterized using transmission electron microscopy, X-ray diffraction, X-ray photoelectron Spectroscopy, cyclic voltammetry, and chronoamperometry. The surfaces of most An nanoparticles were covered with bimetallic PtRu overlayers with a thickness of 1-2 monolayers. The thin PtRu overlayers represented distinct CO stripping characteristics. which may be attributable to the unique surface structures of the PtRU overlayers on the An nanoparticles. PtRU utilization was enhanced by as much as two times compared to that of PtRu/C. which can be attributed to the PtRu overlayers that were deposited only on the surface of the An nanoparticles. The distinct CO stripping characteristics and the enhanced PtRu utilization affected the electrocatalytic activities of methanol oxidation. Pt2Ru1 overlayers; exhibited the highest CO tolerance and the highest methanol oxidation activity. The unique electrocatalytic characteristics of the PtRu overlayer structures on the An nanoparticles are expected to provide methods for reducing the use of active elements. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3183887] All rights reserved.