The synergistic effect of Co alloying and oxidation treatment induced progressive enhancement on oxygen reduction reaction (ORR) activities of Pd/C catalysts is studied. Based on the XPS characterization, a new term, degree of surface oxidation (DSO), is proposed to illustrate their relationship between ORR activity and surface oxidation extent. TPR characterization also provides the evolution of surface species within the topmost region. It can be obviously found the optimal temperature for the promotion of ORR activity on various oxidized samples is 520 K. On the other hand, various heat treatment atmospheres (H(2) and CO) are applied on Pd Co system without changing their particle size. It is clearly evident that the oxidized catalysts can exhibit the superior performance relative to that of the non-oxidized ones, confirming the improved ORR activity is solely ascribed to the formation of surface PdO species with 100% DSO value rather than large particle size effect. Moreover, an explainable model is demonstrated to illustrate the promotional effect of ORR performance on the oxidized PdCo/C catalysts. Copyright (C) 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.