A new method has been developed to determine the probability of miscibility in binary polymer blends through hydrodynamic interaction. This is achieved by the measurement of the free volume content in blends of carefully selected systems-styrene acrylonitrile (SAN)/poly(methyl methacrylate) (PMMA), PMMA/poly(vinyl chloride) (PVC), and PVC/polystyrene (PS)-with positron annihilation lifetime spectroscopy. The free Volume content can predict the miscible/ immiscible nature of the blends but provides no information on the extent of miscibility for different compositions of the blends. We have generalized a model used to understand the viscometric behavior of polymer/solvent systems to polymer/polymer systems through the free volume approach. This model provides two important parameters: a geometric factor (gamma) and a hydrodynamic interaction parameter (alpha). gamma depends on the molecular architecture, whereas of, accounts for the excess friction at the interface between the constituents of the blend, and we propose that alpha can serve as a precursor to miscibility in a system and indicate which composition produces a high probability of miscibility. The efficacy of this proposition has been checked with measured free volume data for the three blend systems. The SAN/PMMA system produces a maximum alpha value of -209 at 20%, PMMA; PVC/PMMA produces a maximum alpha value of -57 at 10% PMMA. Interestingly, for the PS/PVC system, alpha is close to zero throughout the entire concentration range. Therefore, we infer that a is perhaps an appropriate parameter for determining the composition-dependent probability of miscibility in binary blend systems. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 577-588, 2009