The phase behavior of poly(methyl methacrylate) (PMMA)/poly(4-vinylphenol) (PVPh) blends was investigated using differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), small-angle X-ray scattering (SAXS), and Fourier transform infrared (FTIR) spectroscopy. Only one glass transition was observed by DSC in both pure states and blends. However, two loss tangent maxima observed in DMTA relaxation spectra indicate the existence of two phases in the blends. Thermal treatments were found to influence both the number of hydrogen bonds and the proportion of both phases in the blends : quick quenching leads to a lower concentration of hydrogen bonds and to an increase in the quantity of the minor phase. This minor phase gives rise to a tan delta peak appearing at low temperature. It has been assigned to domains composed of isotactic-like PMMA segments and PVPh (phase-i). The T-g determined by the major loss tangent maximum exhibits positive deviation from linearity in the T-g-composition curve and is likely due to the domain of syndiotactic-like PMMA. sequence with PVPh (phase-s). Using DMTA and X-ray scattering measurements, it is estimated that less than 2% phase-i with average domain size 30 Angstrom is dispersed in a phase-s matrix. This type of approach could be used to characterize subtle differences in the behavior of optically active hydrogen-bond-forming polymer blends related to tacticity.