The composition of the Cu precipitate formed during the liquid-phase oxidation of benzene to phenol with the Cu catalyst in the presence Of O-2 and ascorbic acid was investigated in order to clarify the cause of the deactivation of the Cu catalyst. The elementary analysis, the IR spectrum, and the XRD pattern of the Cu precipitate were measured in order to elucidate the composition and the structure. The formed Cu precipitate was confirmed to be a Cu oxalate analogue (Cu[OC-O)Q-O)OH]) based on the similarities of its IR spectrum, XRD pattern, and the elemental analysis with those of copper oxalate. The Cu precipitate was not formed in the absence of ascorbic acid and/or O-2. The Cu/ascorbic acid mole ratio affected the formation of the Cu precipitate. The time dependence of the formation of the Cu precipitate revealed that the Cu oxalate analogue was first produced via the cuprous oxide and then the copper metal. (C) 2004 Elsevier B.V. All rights reserved.