AbstractHigh molecular weight plasticizers can be used if they have a lowTgand are miscible with PVC. For example, linear polyesters exhibit miscibility with PVC when their [CH2]/[COO]ratio is intermediate; in that range, as “miscibility window” has been found. However, the degree of miscibility of miscible polymer blends vary with the structure of the polymers involved and thier concentration. The miscibility of these systems is often assessed by the measurement of a singleTgas a function of composition. A careful examination of experimental data of polyester/chlorinated polymer blends, as well as the use of the free volume theory, indicates that several of these systems exhibit a cusp as a function of composition, which is characterized by a critical volume fraction and a critical temperature. Specific examples are gi