Continuous catalytic wet air oxidation was investigated as a suitable treatment of p-hydroxybenzoic acid chosen as a phenolic compound typically found in olive mill wastewater. The reaction was conducted in a continuous reactor at 140 or 150 C and 50 bar of air using ruthenium catalysts supported by aerogel mixed oxides (Ru/CeO2–Al2O3, Ru/CeO2–TiO2). The influence of the Ru precursor and the nature of the support were studied. The results show that supported ruthenium catalysts were active to oxidize p-hydroxybenzoic acid and that the catalytic activity is very stable.