The compositions in equilibrium two-liquid phases of polymer–solvent mixture systems have been studied from the standpoint of preferential adsorption of one solvent component by measuring φout, νv, and V/m for toluene–isobutanol–polystyrene systems. Here φout is the volume fraction of toluene in the solvent mixture outside the adsorbed regions surrounding polymer molecules; ν′ is the volume of hypothetical toluene layers per unit weight of polymer substance, when the adsorbed regions are divided into pure toluene layers and the remaining parts with solvent composition equal to φout, keeping the amount of toluene in the adsorbed regions unchanged; and V/m is the volume of solvent per unit weight of polymer substance. ν′ is observed to be equal for both the polymer-poor and polymer-rich phases and independent of polymer concentration, so far as the polymer concentration in the latter phase is smaller than 0.2 g./cc. solvent, φout is found to be also equal for the both phases. These observations show that the ternary system at a temperature is approximated by a binary system consisting of polymer molecules which are composed of true polymer molecules and the toluene layers surrounding them, and solvent mixture outside the toluene layers. When V/m and ν′ in the polymer-rich phase are plotted against decreasing φout, ν′ begins to decrease at φout = 0.6, where the change of V/m along a steep curve for φout > 0.6 is slowed down. This suggests that after the polymer-rich phase is deprived of most of the solvent outside the adsorbed regions the adsorbed regions begin to lose solvent. The magnitude of the adsorbed regions per unit weight of polystyrene in a dilute solution with φout > 0.6 is estimated to be about 5 cc./g. from V/m of a polymer-rich phase with φout = 0.6.