The paper deals with the reactivity of RuO2/CeO2 prepared by a wet reduction method: ruthenium was supported on Ce(OH)(3), which was precipitated by alkali-hydrolysis of Ce(NO3)(3), under a reduction condition with formaldehyde at pH 11 and transformed into RuO2/CeO2 catalysts by calcination in air at 773 K. The catalysts were investigated with an ESR technique and were tested for oxidation of propylene. They had no ESR signals by themselves at room temperature, but gave two sets of anisotropic signals upon contact with propylene. These signals were derived from the reaction products between Ru=O and propylene. The intensities of the signals were kept unchanged at room temperature for more than I h in the absence of excess propylene. The signals decayed in the presence of excess propylene and the upfield signal decayed more rapidly. A prior heating of the catalyst in air at 473 K or above caused the increase in the intensity of the upfield signal. The time course of the signal changes discriminated between ethylene and olefins with allylic hydrogen toward Ru=O species. (c) 2005 Elsevier B.V All rights reserved.