A new criterion for evaluating different plasticizers the ability of suppression of the Β transition in plasticized PVC blends. Accordingly, the Β suppression ability is proportional to the PVC-plasticizer compatibility, expressed either by the critical solution temperature, CST, or by the interaction parameter related to the difference between the solubility parameters of the blend components. The criterion is, however, valid for low plasticizer contents (<5%w/w) only, as long as the Β transitions are not overlapped by the α transitions, shifted towards lower temperatures due to the effect of the plasticizer. For higher plasticizer contents the α transition starts to overlap the Β transition and the Β suppression ability of the plasticizer depends increasingly on the efficiency of the plasticizer i.e. on the depression of the glass transition temperature of PVC (related to theTg of the plasticizer). Accordingly, plasticizers with both good efficiency (lowTg) and compatibility are more effective in the Β suppression than plasticizers which have only a higher compatibility but also a highTg (i.e. reduced efficiency).