The concentration power equation recommended by Brekner, Schneider and Cantow for adapting the compositional dependence of the glass temperature, T-g, of compatible binary polymer blends has been used to correlate the experimental T-g data obtained for PVC-plasticizer systems. It is demonstrated that the T-g VS composition data for such systems can be adapted with the condition that two different concentration power equations are applied separately to the two branches of the curves separated by the cusp typical for these systems. Accordingly, different values are obtained for the parameters of the concentration power equation for the two branches of the T-g VS composition curves. A certain significance is attributed to the cusp, characterized by a critical composition of the system. Below the critical composition the plasticizer will be fixed by energetic driven hetero-interaction preferentially in the ’interdomains’ within the amorphous PVC matrix, causing a stiffening of the blend. Above the critical concentration the plasticizer will still penetrate these domains, but this time due to plasticizer-plasticizer homocontacts which thus contribute to an increase of the mobility and consequently of the entropy of the blend. This different behaviour is characterized by means of the different parameters of the concentration power equations for the two branches of the T-g vs composition curves. Additionally it is shown that the parameters can be correlated with the usual compatibility criteria of the plasticizer ranking.