In this investigation, we have studied the adsorption of benzoic acid from mixed solvents of carbon tetrachloride-benzene, carbon tetrachloride-ethyl alcohol, benzene-ethyl alcohol on sugar charcoal and silica gel. From mimed solvents of carbon tetrachloride-benzene, the adsorption of benzoic acid varied monotonically with the composition of mimed solvents, and a negative deviation from straight line is observed. From mixed solvents of carbon tetrachloride-ethyl alcohol and benzene-ethyl alcohol, either by adding small amount of ethyl alcohol into carbon tetrachloride (or benzene) or vice versa, the adsorption of benzoic acid always decreased sharply. The results of sugar charcoal and silica gel are similar qualitatively. Based on the assumption of competitive effects of solvents and solutes on the surface, the results may be expained.