A solvent for polystyrene resin treatment is used for reducing the volume of a polystyrene resin by contacting it with the solvent; and a method of treating polystyrene resins. The solvent is characterized by comprising 20 to 70 wt. % of diethylene glycol dimethyl ether and 30 to 80 wt. % of C.sub.9-13 aliphatic hydrocarbon. The method employs the solvent.