The phenomenon of antiplasticization was examined for poly-vinyl chloride containing 10 phr tricresyl phosphate, by means of dynamic mechanical tests and with constant strain rate equipment, using both tension and plane-strain compression deformation modes. While the activation energy for both α and β relaxations was decreased by the addition of the plasticizer, the magnitude of β dispersions decreased and the modulus increased over a temperature range lying between the α and β transitions of the plasticized polymer. Similar changes were found with respect to the yield stress measurements, from which a contour diagram was drawn showing the strain rate/ temperature relationship of antiplasticization. The values of activation energy for yield phenomena were found to lie between those obtained for the α and β relaxations. The activation volume, on the other hand, was found to increase over the same temperature range in which antiplasticization is observed.