We report on the morphological variations of SP/H blends containing poly(styrene-block-2-vinylpyridine) diblock copolymer (SP) (M-w = 130K) and poly(4-hydroxystyrene) (H) (M-w = 14K) with different blend compositions. The structures were observed by transmission electron microscopy (TEM) as well as by small-angle X-ray scattering (SAXS). The results were compared with those obtained from SP/P blends, where P denotes the poly(2-vinylpyridine) homopolymer. The neat SP diblock copolymer, with an S block volume fraction. 0, of 0.67. has a hexagonally packed cylindrical structure. As P (M-w of 6.6K) is added to the copolymer, morphology of the blend transits to a lamellar structure. Further addition of the homopolymer Causes macrophase Separation of P homopolymer chains. In contrast, macrophase separation of H homopolymer has never been observed in SP/H blends, even upon addition of large amounts of H. At first, it was noted that file Unusual morphological transition from the cylindrical structure to the spherical Structure Occurs upon addition of H. Subsequently transitions to cylindrical. lamellar and, finally to inverse cylindrical structures occur With successive additions of H. This result is due to the miscibility of P and H via hydrooen-bonding interactions which appear to serve as the driving force for production of uniform nanophase-separated Structures covering I wide composition range.