Abstract: Liquid-phase oxidation of toluene by air is an environment friendly technology to produce benzoic acid.In SNIA benzoic acid process, two heat exchangers are used to remove the reaction heat.However, severe scaling often occurs, especially on the tube surface of the exchangers.In the present study the scale was detected to be precipitates of CoC₂O₄&#8226;2H₂O, which was formed from the catalyst Co ion and the by-product oxalic acid.In the oxidation intermediate products of toluene, oxalic acid, maleic acid and hydroquinone were detected by HPLC.According to the research results,oxalic acid was formed through the deep oxidation of toluene or its derivates.Reaction temperature, pressure, air flow and the contents of benzene, water, benzoic acid, as well as the catalyst amount all affected scaling significantly.It was observed that the factors which inhibited the oxidation to benzoic acid enhanced scale formation.Aeration rate influenced the scaling reversely.Impurities, such as benzene or water, increased the scaling amount.Scale decreased with increasing Co content(<100 μg&#8226;g^-1).Less effect of reaction pressure was observed.