Ruthenium oxide-like (RuxOy) nanoparticles were prepared by the decomposition of carbonyl precursors in organic solvents under mild conditions. CO oxidation over the reduced particles was studied by combined in Situ X-ray diffraction and gas-phase analysis. For a sample prepared in dichlorobenzene, in a flow of CO/O-2 = 0.55, oxidation of CO started at ca. 70 degrees C with the simultaneous formation of amorphous surface oxide. On further heating. the CO? production increased proportionally with the rate of formation Of Surface oxide. Below ca. 130 degrees C the catalyst can be reversibly reduced in CO to the disordered precursor state, average size ca. 2.8, run. After catalytic performance for 3 It at 180 degrees C, reduction in CO of partially oxidized RuxOy is accompanied by a burst of CO2 partial pressure and induces particle growth to similar to 5 run due to the exothermic heat of the reaction. These results are comparable to earlier results for ruthenium single-crystal surfaces studied under low pressure conditions. (c) 2005 Elsevier Inc. All rights reserved.