Phenylethyl-substituted poly(p-xylylene) (PEPPX) was prepared by base-induced 1,6-dehydrohalogenation of the corresponding 4-chloromethyltoluene. Very high degrees of polymerization of about 3150 were obtained already at low conversions. Several standard retarders for radical polymerizations failed to limit the degree of polymerization significantly. However, best control was accomplished by the addition of dibenzoyl peroxide. The degree of polymerization of PEPPX was depressed to about 110 by addition of 10-15 mol-% of dibenzoyl peroxide.