Al2O3 supported transition metal (Mn, Fe, Co, Ni, and Cu) oxide catalysts were prepared and tested for the wet oxidation of phenol. The supported copper oxide catalysts showed the highest catalytic activity due to their highest surface reducibility. There was carbonaceous deposits on the used catalysts for the wet oxidation of phenol and the supported manganese oxide catalysts showed the highest amount of carbonaceous deposits. These carbonaceous deposits must have their own micropores which resulted in the decrease of the pore volume and the increase of the surface area. The NMR and FTIR spectroscopy showed that the carbonaceous deposits were mostly of aromatic nature and contained some oxygen-bearing groups such as carboxylic acids and alcohols.