Miscibility and molecular interactions of poly(ethylene-co-propylene), EPC, and a series of homologous poly(alkyl methacrylates), PAMA, wnamely: poly(methyl methacrylate) (PMMA), poly(dodecyl methacrylate) (PDDMA) and poly(octadecyl methacrylate) (PODMA)x were thoroughly investigated by dilute solution viscometry in xylene as a common solvent at 30 8C. Intrinsic viscosities and viscometric interaction parameters were experimentally determined for binary (polymeryxylene) and ternary (EPCyPAMAyxylene) systems. Since all investigated EPCyPAMA binaries exhibited negative viscometric interaction parameter values (Db -0), the existence of repulsive intermolecular interaction was deduced and EPCyPAMA blends were 12 assigned to be immiscible. The conclusion was supported by calculations employing the group contribution approach of Coleman, Graf and Painter. The calculated values of interaction parameters for (co)polymer blends, L , were 7.60 J cm , 1.63 J cm y3 y3 12 and 1.15 J cm for EPCyPMMA, EPCyPDDMA and EPCyPODMA, respectively. According to the viscometric and blend y3 interaction parameter values, the immiscibility of polymeric components decreases in the following order: EPCyPMMA4EPCy PDDMA)EPCyPODMA. The values of Huggins' constants in xylene solutions were found to be relatively high, i.e. 1.8, 1.2 and 2.0 for PMMA, PDDMA and PODMA, respectively, pointing to a certain degree of self-association between polymer molecules. However, for EPCyPAMAyxylene systems those values decreased sharply with the increase of EPC content in the ternaries. The superposition of the two effects: (1). repulsive interaction between EPC and PAMA molecules; and (2). self-association of PAMA molecules (affected by the dilution upon introduction of EPC) was proposed to influence the viscosity of investigated systems as a function of polymer mixture composition. The approach of Bohdanecky and Kovar was used for the estimation of self-´ǎ ssociation constants of PAMAs in xylene. The same approach was extended to ternary systems; in that form it was able to account for the compositional variation of viscometric interaction parameters.