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.