The mixed critical hemimicelle concentrations of dodecyl benzenesulfonate and dodecyl sulfate adsorbed on alumina from an aqueous solution have been determined using zeta potential measurements. The critical micelle concentrations of aqueous solutions of mixed surfactants have been determined using surface tension measurements. It is found that whereas mixed micelle formation shows ideal behavior, the mixed hemimicelle formation shows positive deviation from ideality. The results have been explained based on the equivalent contribution of 3.5 methylene groups by the aromatic benzene ring in dodecyl benzenesulfonate. Rubingh theory has been applied to the formation of the hemimicelle.