We describe morphological variations in binary mixtures of a poly(styrene-b-2-vinylpyridine) diblock copolymer (SP) (M-w = 151 K) and poly(4-hydroxystyrene)s (M-w = 8K, 14K, 52K) (H8, H 14, H52) as a function of Molecular weight of homopolymer H and as a function of blend ratio. Transmission electron microscopy reveals ordered nanophase-separated structures for all samples regardless of molecular weight and the amount of homopolymer added. As much as 28-fold homopolymer H52, which has it M-w higher than that of the P block, call be added to the SP block copolymer without inducing macrophase separation. This unusual miscibility is apparently due to the affinity of P for H via hydrogen-bonding interactions. Furthermore, at a constant volume fraction of polystyrene (phi(s) of 0.48) and an H/P ratio of 3.2, it structural transition from hexagonally packed cylinders to lamellae occurs upon increasing the M-w of the homopolymer. The small-angle X-ray scattering (SAXS) data obtained for these blends show that the intercylinder distance increases as the M-w of homopolymer increases from 8K to 14K. This Finding may imply that the low-M-w homopolymer (H8), which is distributed uniformly in the P microdomain, leads to ail increase in the distance between block copolymer junction points, while chain localization of added H homopolymer may begin at SP/H14. Furthermore, the high-M-w homopolymer (H52) appears to be weakly segregated in the middle of the P domain. This constrains the homopolymer effect on the blocks chain conformation and oil the distance between junction points and prevents the formation of the cylindrical structure when molecular weight of H increases. Thereby, the simple lamellar structure is maintained for SP/H52 blend at phi(s) of 0.48. It should be noted that the SP/H52 blend forms ail ordered lamellar structure even at a M-w(H)/M-w(P) ratio of 1.5.