Binary and ternary blends of polystyrene-block-poly(p-hydroxystyrene) (MW = 1 x 10(4)-1 x 10(4)) with various homopolymers were studied. In binary blends, in which the homopolymers poly(ethylene oxide) (PEG), poly(p-vinylpyridine) (PVPY), and poly(n-butyl acrylate) (PnBA) had attractive interactions via hydrogen-bonding with the poly (p-hydroxystyrene) block but were immiscible with the polystyrene block, microphase separation played a dominant role in morphology development. The attractive interaction parameter appeared to be the major factor that influenced the phase separation mechanism. The results obtained with three different molecular weights of PEO suggested that the molecular weight effect was not important when the attractive interaction parameter was sufficiently strong. In PVPy/copolymer blends, microphase separation was also prevalent, but phase separation in PnBA/copolymer blends seemed to follow the macro-micro mechanism. In blends of the diblock copolymer with poly(vinyl methyl ether), which was miscible with both blocks, the latter acted as a polymeric solvent for the copolymer and gave a single-phase mixture when present in sufficient amount. The addition of the block copolymer reduced the domain size in PS/poly(ethyloxazoline) blends. In PS!poly(methyl methacrylate) and PS/poly(n-butyl methacrylate) blends, the morphology changed to a cocontinuous pattern upon incorporation of the copolymer.