Ruthenium catalysts prepared by ion exchange of active carbons and high-surface-area graphites are active for the wet air oxidation of aqueous solutions of acetic acid (5-20 g/liter). A total conversion into CO2 can be achieved between 448 and 473 K using air as oxidizing agent. No leaching of ruthenium can be detected which indicates that the reaction proceeds on the heterogeneous catalysts. For the same particle size (1 nm), graphite-supported ruthenium catalysts are much more active (up to 0.4 mol h(-1) g(Ru)(-1) at 473 K in a stirred batch reactor pressurized with air at 10 MPa) than active carbon-supported catalysts. The lower activities of the latters could be due to internal diffusion limitation since the I-nm Ru particles are located inside the micropores. However, graphite-supported catalysts might be intrinsically more active because of an electron transfer from graphite to metal particles which would increase the resistance of ruthenium to oxygen poisoning. It was also shown that the activity of ruthenium is particle size dependent : the smaller the sizes, the lower the activities. This effect could be interpreted by the higher adsorption energy of oxygen on the small particles which produces a poisoning of the metal surface. From measurements of the reaction rates on the Ru/HSAG graphite catalyst at different temperatures, pressures, and acetic acid concentrations, it was established that the reaction orders were zero and 0.65 with respect to the concentration and oxygen pressure, respectively; the activation energy of the reaction was 100.5 kJ mol(-1). An equation describing the reaction kinetics was proposed.