Manganese/cerium composite oxide (MnO2/CeO2) proved to be a potent solid catalyst for the catalytic wet oxidation (CWO) of aqueous phenol under mild treatment conditions (80-130 degrees C, 0.5 MPa O-2 pressure). Despite the fact this catalyst exhibited an important activity in eliminating completely phenol and total organic carbon (TOC), its poor selectivity to CO2 was demonstrated as a result of deposition on the catalyst surface of carbonaceous deposits with high carbon content. Promotion of MnO2/CeO2 with platinum improved the CO2 yield of phenol oxidation. Evidence for the reduction of the carbonaceous deposit over Pt-MnO2/CeO2 was confirmed via elemental analysis and temperature-programmed oxidation (TPO). Surface analysis by X-ray photoelectron spectroscopy (XPS) indicated that the deposits built up more on cerium than on manganese and that platinum increased the aromaticity of the deposit.