A well-alloyed and highly CO-tolerant 45 wt % PtRu/C catalyst was prepared by sequential mixing of the precursors in a polyol method. The alloying degree of the PtRu/C catalyst was much higher than that of the conventional PtRu/C catalyst prepared by simultaneous mixing of the precursors in the polyol method. In the sequential mixing approach, Pt and Ru compounds were reduced at the same time because Ru3+ was partly reduced to Ru2+ before the addition of H2PtCl6, which promoted PtRu alloy formation, and subsequent reduction treatment by ethylene glycol at a high temperature further improved the alloying degree. Greater CO tolerance of the well-alloyed PtRu/C catalyst was verified by a negative shift in peak potential of COads electrooxidation in CO stripping voltammetry. The well-alloyed PtRu/C catalyst showed a better single-cell performance than the conventional and commercial PtRu/C catalysts when the anode was fed with CO-contaminated H-2. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3223666] All rights reserved.