We have investigated the phase behavior or poly(4-vinylphenol-b-styrene) (PVPh-b-PS) when respectively blended with poly(4-vinylpyridine) (NVP), poly(methyl methacrylate) (PMMA), and PVPh homopolymers by mediated hydrogen bonding strengths with the PVPh block of the copolymer. The Fourier transform infrared spectroscopic result indicates that the PVPh-b-PS/P4VP blend has a much higher fraction (f(H)) of hydrogen-bonded PVPh blocks for it significantly higher miscibility compared with the blends with PMMA and PVPh homopolymers. Consequently, the PVPh-b-PS/P4VP blend, behaving as a neat diblock copolymer, exhibited a series of order-order phase transitions from the lamellar, gyroid, hexagonally packed cylinder to body-centered cubic structures when the P4VP content increases from 6 to 71% (volume fraction), as evidenced consistently by transmission electron microscopy and small-angle X-ray scattering. In contrast, both file PVPh-b-PS/PMMA and PVPh-b-PS/PVPh blends maintained essentially the lamellar structure upon a similar volume fraction increase in the homopolymers; the lamellar structure, however, was distorted to different extents at higher volume fractions of the additives, depending on the hydrogen bonding strength. On the basis of the results, the ratio of interassociation equilibrium constant (KA) over self-association equilibrium constant (K-B), K-A/K-B, is introduced as a convenient guide in estimating the phase behavior of similar polymer blends featuring hydrogen bonding interactions between file homopolymer additive and copolymer: with a K-A/K-B ratio much larger than unity, the blend system tends to behave as a neat copolymer; with a K-A/K-B ratio significantly smaller than unity, phase separation instead of order-order phase transitions call be expected for the blend above certain volume fraction of homopolymer additive.