Positive deviations of the modulusEband yield strengthSybof polymer blends consisting of partially miscible polymers are analyzed by combining models for miscible and immiscible blends. The effect of the composition of binary blends on the composition of conjugate phases over the entire composition range is described by an empirical formula that requires two parameters extractable from experimental data. Positive deviations ofEbandSybare ascribed to two effects: (1) respective properties of one or both conjugated phases are higher than those of parent polymers; (2) molecular mobility in both conjugate phases is reduced due to associative interactions (heterocontacts) between the chains of components. These effects are then taken into account in the predictive scheme, which combines (1) an equivalent box model (EBM) and (2) data on phase continuity rendered by the equations proposed by the percolation theory. Three typical situations are considered in model calculations: (1) components do not show any miscibility and interaction; (2) components are partially miscible, but their interaction in conjugate phases is negligible; (3) conjugate phases consist of strongly interacting components. The scheme satisfactorily fits available experimental data if associative interactions are encompassed by means of an adjustable interaction parameter.