Scaling is unavoidable during hydrocarbon oxidation catalyzed by metal salts, such as the liquid-phase oxidation of toluene to produce benzoic acid. Scaling often results in many operation problems, such as catalyst loss, heat transfer efficiency reduction, and pipe blockage. In this paper, a scale sample was collected from a commercial benzoic acid device in which cobalt acetate was used as the catalyst, and its composition was determined. Furthermore, reaction conditions affecting scaling were investigated and the mechanism of scaling was discussed. The results show that the scale is a precipitate Of CoC2O4.2H(2)O. Its composition is Co, 27.3% (Wt %); C2O42-, 41.4% (wt %); CH3COO-, 8.5% (wt %); residues, 4.1% (wt %); and others, 2.7% (wt %). The formation of oxalic acid results in the precipitate of cobalt ion. The oxalic acid forms through a phenol route, which is a competing reaction with the formation of benzoic acid.