This work presents results of the studies of the CO tolerance of Pt/C, Ru/C, Pt/C + Ru/C and PtRu/C forming different electrode assemblies for the hydrogen oxidation reaction (HOR) in polymer electrolyte fuel cells. Anodes consisting of the physical mixtures of Pt/C + Ru/C in two distinct configurations were used to elucidate the Ru effects on the HOR electrocatalysis of Pt in the presence of CO. The results showed that at low anode overpotentials the CO tolerance for the PtRu/C catalysts is promoted by an electronic effect of Ru on Pt, while at high overpotentials, the CO tolerance is enhanced by the occurrence of the water splitting reaction and the CO oxidation taking place via the Langmuir-Hinshelwood mechanism. The results also demonstrate the occurrence of spontaneous oscillatory behavior of the anode potential and the oscillatory production of CO(2), as monitored by on-line mass spectrometry (EMS) at the anode gas outlet. Evidences of the occurrence of a CO methanation reaction under the fuel cell environment are also given. (C) 2009 Elsevier B.V. All rights reserved.