Influence of metal salts on the direct synthesis of phenol by hydroxylation of benzene with oxygen and hydrogen in the liquid phase was investigated for a heterogeneous platinum catalyst supported on silica. Promotive effects were observed with acetylacetonates of vanadium, iron, lanthanum, and yttrium, while with the other elements, the rate of phenol formation was retarded. Among the preferable compounds, vanadium(III) acetylacetonate (V(acac)(3)) produced the most beneficial effect. The highest rate of phenol formation was obtained at ca. 20 ppm of V(acac)(3). Among various vanadium compounds studied, the vanadium compounds having V=O seemed to have the promotive effect. It was estimated that oxygen activated on the vanadium species was transferred to platinum site and as a result, the rate of phenol formation was promoted. (C) 2002 Elsevier Science B.V. All rights reserved.