The liquid-phase oxidation of benzene to phenol using a Cu catalyst supported on Al 2 O 3 was studied at 303 K in an aqueous solvent with a variety of concentrations of acetic acid. Gaseous O 2 and ascorbic acid were utilized as an oxidant and a reducing reagent, respectively. An aqueous solvent with more than around 70-80 vol.% acetic acid concentration was confirmed to cause a sharp apparent decrease in the Cu leaching from a Cu catalyst (Cu/Al 2 O 3 ) impregnated on Al 2 O 3 , as well as a CuO-Al 2 O 3 catalyst prepared by a co-precipitation method reported previously, during the benzene oxidation, keeping the yield of phenol comparatively high. The apparent Cu leaching was largely inhibited in an aqueous solvent with high acetic acid concentration in comparison with a solvent of low acetic acid concentration, irrespective of the presence of benzene and/or ascorbic acid. The influences of both the Cu amount supported on the Cu/Al 2 O 3 catalyst and the amount of ascorbic acid on the yield of phenol were investigated in the aqueous solvents with acetic acid concentrations of both 5.7 and 80 vol.%. The Cu/Al 2 O 3 catalyst in the aqueous acetic acid solvent was deactivated in the comparatively initial stage of the oxidation of benzene. The catalytic activity of the used Cu supported catalyst for phenol formation was almost completely recovered by calcining the catalyst at 773 K. The amount of the deactivated Cu species based on a XRD peak at 2ฮธ = 23 โข in the used Cu/Al 2 O 3 catalyst was found to have a correlation with the yield of phenol.