The effects of Ce addition on the activity of Pt/Al2O3 catalysts were investigated for the wet oxidation of phenol. Platinum was impregnated on gamma-Al2O3 by using two different precursors: one is anionic (i.e., H2PtCl6) and the other is cationic (i.e., Pt(NH3)(4)Cl-2). The Pt catalysts from the former showed much higher activity than that from the latter, because the former resulted in a better metal dispersion than the latter. Cerium addition lowered the catalytic activity of Pt/Al2O3 catalysts from H2PtCl6, while it improved the activity of Pt/Al2O3 catalysts from Pt(NH3)(4)Cl-2. The effect of Ce addition on the wet oxidation activity of Pt-Ce/Al2O3 could be explained by the differences in Pt dispersion and the Pt-Ce interactions.