The peroxide crosslinking of plasticized poly(vinyl chloride), PVC, blended with trimethylolpropane trimethacrylate, TMPTMA, has been studied by following gel formation and changes in mechanical properties. By applying the right preparation conditions and peroxide type, it was possible to avoid crosslinking while dry-blending the materials. During compression moulding, gel yield was found to increase with increased peroxide and TMPTMA concentrations and with curing time and temperature. The crosslinking process was found to be comprised of two primary reactions, a rapid polymerization of TMPTMA followed by grafting of the TMPTMA onto the PVC backbone. In the absence of either peroxide or methacrylate monomer no crosslinking was observed. Crosslinking was found to be independent of the tribasic lead sulfate stabilizer level and contrary to what is expected the crosslinked sheets showed excellent thermal stability. The softening temperature of the cured sheets increased with crosslinking.