A new method to characterize individual interfaces in ternary polymer blends from experimentally measured fractional free volume from Positron Annihilation Lifetime Spectroscopy (PALS) has been developed. By this, we derive the composition dependent miscibility level in ternary polymer blends. This method has its genesis in KRZ (Kirkwood-Risemann-Zimm) theory which introduces hydrodynamic interaction parameter as a measure of excess friction generated at the interface between dissimilar polymer chains resulting in energy dissipation. The method successfully applied for binary blends has been theoretically modified to suit ternary blends in the present work. The efficacy of this method has been tested for two ternary blends namely polycaprolactone/poly(styrene-co-acrylonitrile)/poly(vinyl chloride) (PCL/SAN/PVC) and polycaprolactone/poly(vinyl chloride)/poly(vinyl acetate) (PCL/PVC/PVAc) in different compositions. We obtained a maximum effective hydrodynamic interaction ((eff)) of -12.60 at composition 80/10/10 of PCL/PVC/PVAc while PCL/SAN/PVC showed -1.60 at 68/16/16 composition. These results suggest that these compositions produce high miscibility level as compared to other compositions. DSC measurements have also been used to supplement positron results. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3335-3344, 2013