One major challenge of PEM fuel cells development is to overcome the activity and durability issues of the current anode materials which are susceptible to hydrogen impurities. To design stable and efficient catalysts with enhanced reformate tolerance, a comprehensive understanding of the underlying mechanisms is crucial. In this work, the CO and CO2 tolerance of a series of Pt-based catalysts are tested in a PEM fuel cell. We report that the CO tolerance is the highest for PtMo/C followed by PtCoMo/C, PtRuMo/C, PtRuPb/C, PtRu/C, PtCo/C, PtFe/C, PtNi/C and Pt/C; while the CO2 tolerance increases in the order: PtCo, PtNi>PtRuPb>PtRu>PtCoMo>PtRuMo>PtFe>Pt>PtMo. In situ XAS measurements in combination with FEFF8 calculations are performed to correlate the CO and CO2 tolerance trends with the electronic properties of these Pt-based alloy catalysts. We find that the anode overpotential in the presence of CO2 can be generally related to the experimental Pt d-band vacancy or calculated d-band center, and thus governed by the Pt electronic properties modified by the alloyed metal(s). No such correlation is observed between the anode overpotential in the presence of CO and Pt electronic properties, which highlights the key roles of Mo or Ru in improving CO tolerance via promotion and bifunctional mechanisms. Building upon these results a new ternary alloy PtCoMo/C was synthesized. This electrocatalyst shows the best reformate tolerance in low temperature PEM fuel cells by taking advantage of the bifunctional mechanism induced by Mo and the ligand effect induced by Co simultaneously. Our findings put forward a theory which gives a strong perspective for further research and development of new inexpensive catalysts with superior CO tolerance and durability. (C) 2013 Elsevier Ltd. All rights reserved.