Permeability to water vapor and oxygen, elastic modulus, tensile strength, and impact strength of polystyrene-polypropylene and high-impact polystyrene-polypropylene blends were determined as functions of blend composition and morphology. Three types of styrene-butadiene block copolymers were tested as compatibilizers and found to improve mechanical properties of blends. The experimental data on permeability and modulus were compared with the predictions for the studied binary and ternary blends. The predictive scheme employs a two-parameter equivalent box model and the data on phase continuity of constituents calculated using general equations derived from percolation theory. Blends with decreased permeability and plausible mechanical properties were proposed with regard to intended applications in food packaging.