The liquid-phase catalytic oxidation of benzene to directly produce phenol was attempted under mild reaction conditions using supported Cu catalysts in aqueous acetic acid solvent. Gaseous oxygen and ascorbic acid were used as an oxidant and a reductant, respectively. Among the supported Cu catalysts studied, the Cu catalysts prepared by the impregnation method, irrespective of the oxide supports, the Cu species were considerably leached during the benzene oxidation. A supported Cu catalyst (CuO-Al2O3) prepared by co-precipitation of Cu(NO3)(2).3H(2)O and Al(NO3)(2).3H(2)O inhibited the leaching of Cu species in comparison with the Cu catalysts supported by the impregnation method on Al2O3, SiO2, MCM-41, etc. The influences of the amount of supported Cu, the partial pressure Of O-2, the amount of ascorbic acid, the concentration of acetic acid in the solvent, and the reaction temperature on the phenol yield were investigated using the CuO-Al2O3 catalyst. The aqueous solvent including high concentration of acetic acid (around 80 vol.%) dramatically inhibited the leaching of Cu species in the CuO-Al2O3 catalyst. H2O2, which is considered to play an important role in phenol formation, was detected during the benzene oxidation catalyzed by CuO-Al2O3.