Catalytic wet air oxidation of an aqueous solution of p-hydroxybenzoic acid chosen as a representative phenolic pollutant of olive mill wastewater was conducted over ruthenium catalysts supported on TiO2 or ZrO2 in a stirred batch autoclave and in a fixed bed reactor at 140 degrees C and 50 bar air. The objective was to examine the stability of these catalysts by conducting catalyst recycling experiments in batch reactor and long-term stability tests in trickle-bed reactor. A small deactivation was observed during the first recycling of ruthenium catalyst in batch experiments, which was totally reversible after catalyst reduction. A small deactivation was also observed at the start of the continuous runs. This moderate deactivation was attributed to an over-oxidation of the surface or subsurface of ruthenium particles. Then the catalysts proved to be very stable in ca. 50-day continuous runs. Noticeably, no leaching from the catalysts was observed at all under these operating conditions. Catalysts prepared on ZrO2 from ruthenium nitrosyl nitrate showed the highest performances. (c) 2007 Elsevier B.V. All rights reserved.