A crosslinking system consisting of 1,1-di-t-butylperoxy-3,3,5-trimethyl cyelohexane peroxide and trimethylolpropane trimethacrylate (TMPTMA) has been used to introduce crosslinks into unplasticized poly(vinyl chloride) (PVC). The influence of the concentration of both reagents has been investigated, and crosslinking monitored by determination of the remaining sample weight after Soxhlet extraction with tetrahydrofuran. The system used (i.e., 0.5-2.0 phr peroxide with 5 to 15 phr TMPTMA) has been shown to be effective for crosslinking PVC. Gel contents of 30-40% have been obtained, premature crosslinking during processing is largely avoided, but thermal stability still needs to be improved. Considerable improvements in elevated temperature mechanical properties can be attained using an appropriate TMPTMA/peroxide concentration. The best tensile properties were obtained with 0.5 phr peroxide and 15 phr TMPTMA. Observed increases in T-g, also achievable with only 0.5 phr peroxide, but only slightly dependent on TMPTMA concentration, represent, a useful increase in service temperature for the resulting compound. Lower peroxide levels may be adequate to achieve property improvements.