AbstractEarlier work reported the superposition of flow curves for plasticized compounds of vinyl chloride homopolymer with corresponding curves for unplasticized resin. Viscosity shift factors for three plasticizer systems plotted against temperature define an apparent fusion temperature for the bulk polymer. A Brabender Plasticorder, used as a temperature‐scanning rheometer, determined fusion temperatures for various plasticized PVC compounds. These data confirm a fusion temperature for bulk polymer near 205°C, and permit calculation of Flory‐Huggins χ parameters over substantial ranges of composition and temperature. The thermodynamic parameter correlates well with viscosity shift factors for compounds with polymer volume fractions below 0.6. The possibility is therefore raised of calculating flow characteristics for plasticized PVC compounds from knowledge of χ only. The temperature and composition dependence of χ, estimated for the system PVC‐diiso‐octyl phthalate, is similar to reported variations in simpler polymer‐diluent systems. Thus, PVC‐plasticizer systems may follow analogous ther