The relationship between the state of Ru on CeO2 and catalytic activity in the wet oxidation of acetic acid was investigated for Ru/CeO2 catalysts prepared by different methods. The temperature programmed reduction (TPR) experiments of Ru/CeO2 showed that the oxygen species of RuO2 was reduced at different temperatures depending upon the methods of preparation. Ru species reduced at low temperatures could not be observed by TEM and XRD. It was concluded that Ru-O-Ce bonds in the well-dispersed Ru species are highly fragile and its mobile oxygen is the active species in the wet oxidation. (C) 2003 Elsevier Science B.V. All rights reserved.