PURPOSE: To produce p-chlorostyrene in high purity and selectivity, by chlorinating a polystyrene in an inert solvent in the presence of a Lewis acid using a compound such as phenoxanthine as a cocatalyst and thermally decomposing the resultant poly-p-chlorostyrene.

CONSTITUTION: Polystyrene is chlorinated in an inert solvent such as chlorobenzene, chloroform, dichloroethane, etc., in the presence of a Lewis acid such as antimony trichloride, aluminum chloride, lead sulfide, etc., in combination with a cocatalyst comprising the compound of formula [X is perfluoroacyl or H; Y is -O- or -NR- (R is H or trifluoroacetyl); n is 0W2] (e.g. 5-trifluoroacetylphenothiazine) and the obtained poly-p-chlorostyrene is thermally decomposed to obtain p-chlorostyrene.

EFFECT: Inexpensive waste-grade polystyrene to be disposed can be used as a raw material.