Poly(ethylene-co-vinyl acetate) compounds and vulcanizates containing dicumyl peroxide (DCP) (HD) were pre­pared, and the curing agent and reaction products were analyzed using gas chromatography/mass spectrometry (GC/MS). Samples containing trially cyanurate (TAC) and DCP (HDT) were also prepared and analyzed. Some raw DCP were decom­posed in the injector region of GC to produce acetophenone. DCP was detected in the HD compound but was not observed in the HD vulcanizate, and instead acetophenone and 2-phenyl-2-propanol were detected. Both DCP and TAC were observed in the HDT compound but not in the HDT vulcanizate, where acetophenone and 2-phenyl-2-propanol were detected. Thus, some of the DCP decomposed during the compounding process. The formation mechanism for acetophe­none and 2-phenyl-2-propanol during the crosslinking reaction was identified, and differences in the crosslinking reactions of HD and HDT compounds were discussed.