Blends of polystyrene/poly (4-vinylpyridine) have been prepared by casting from a common solvent. The compatibility of the blends was studied by using dilute solution viscometry (DSV), differential scanning calorimetery (DSC), Fourier transformation-infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM). The relative viscosity versus composition plots for the blends are not perfect linear. The corresponding intrinsic viscosity values show negative deviation from ideal behavior when plotted against composition. Also, the modified Krigbaum and Wall interaction parameter, Delta b, shows small and negative values for all compositions except for the blend PS/P4VP (25 : 75). The results indicate that the polymers are incompatible but small interaction values predict physically miscible blends which eventually show phase separation, as is observed in the present studies. However, the blends as obtained show a single, composition-dependent, glass transition temperature that fits the Fox equation well, indicating the presence of homogeneous phase. The constant, k obtained from Gordon-Taylor equation suggests intermolecular attraction between these polymers. FT-IR and SEM support the results of DSV and DSC. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 1729-1735, 2011