The effect of co-alloying of Co and Cr was investigated by synthesizing and analyzing Pt3M and PtM (M=Co and/or Cr) catalysts for the oxygen electro-reduction reaction (ORR) activity. In the Pt3M catalysts, we could not observe significant differences among the alloy catalysts before and after annealing, except for the high specific activity of a Pt3Co/C-900 catalyst (Pt3Co/C catalyst annealed at 900 degrees C under H-2/Ar (5.2 mol% H-2) flow for 5 min.). The electrochemical properties of the alloy catalysts were much different in the PtM catalysts. After annealing at 900 degrees C, the specific activities of the PtM catalysts increased in the order of PtCo/C-900 (5000 mu A cm(pt)(-2)) > PtCo0.5Cr0.5/C-900 (2400 mu A cm(pt)(-2)) > PtCr/C-900 (1080 mu A cm(pt)(-2))> Pt/C (334 mu A cm(pt)(-2)). On the other hand, ORR mass activity exhibited a different order of PtCo0.5Cr0.5/C-900 (132 mA mg(pt)(-1)) > PtCr/C-900 (129 mA mg(pt)(-1)) > PtCo/C-900 (37 mA mg(pt)(-1)). The low mass activity of the PtCo/C-900 catalyst was explained by small electrochemically active surface area (EAS). The intermediate specific ORR activity and EAS values of the PtCo0.5Cr0.5/C catalyst shows that the two opposite characteristics of PtCo (low mass activity and high specific activity) and PtCr (high mass activity and low specific activity) were combined by co-alloying of Co and Cr with Pt. In addition, this result shows that the electrochemical properties of the ternary catalyst can be tuned by controlling the composition to achieve high mass activity. (C) 2012 Elsevier Ltd. All rights reserved.