The support effect on the low temperature catalytic combustion of methane over palladium catalysts was studied by using a series of metal oxides as the support. The catalytic activity varied with the support, and support oxides with moderate acid strength gave maximum methane conversion. The oxidation state of palladium after the catalytic run, characterized by XRD and XAFS, also varied with the support, and it was higher on more basic supports. The relation between the catalytic activity and the oxidation state derived from these results did not agree with that obtained by changing oxygen partial pressure. The catalytic activity increased with oxygen partial pressure even up to that corresponding to 30 times as high as the stoichiometric ratio, indicating that the activity was higher at higher oxidation state. This agreed with lower activity of palladium supported on acidic support, but not with that on basic support. The lower activity of palladium supported on basic support in spite of higher oxidation state was attributed to excess stabilization of palladium oxide cluster anion such as (PdO)(n)(delta-) by electrophobic cation in a similar way to the formation of binary oxide from PdO and basic support at the boundary of them. Lower reactivity of oxidized form of palladium was confirmed by TPR by methane and TPD of oxygen, and the formation of binary oxide containing palladium and magnesium in Pd/M-O catalyst was confirmed by Pd K-edge XAFS study. (c) 2006 Elsevier B.V. All rights reserved.