Interactions in binary polymer systems have been analysed from the viscosity behaviour. Dilute solution viscosity measurements were made on three different ternary polymer (1)/polymer (2)-solvent (3) systems, i.e. acrylonitrile-butadiene rubber (NBR)/polystyrene, NBR/poly(styrene-co-acrylonitrile), NBR/poly(acrylonitrile-co-butadiene-styrene) in chloroform, as a function of blend composition. The polymer-polymer interaction parameter chi(i(p-p)) and the corresponding polymer-solvent interaction parameter chi(i(p-s)) were determined for ail the blend systems. In order to predict compatibility, the interaction parameter term Delta b, obtained from modified Krigbaum and Wall theory, and the difference in the intrinsic viscosities of the polymer mixtures and the weight-average intrinsic viscosities of the two polymer solutions taken separately are used. These results were correlated with the experimentally determined densities and were found to be in agreement. Finally, the results are correlated with spectroscopic evidence.