The miscibility of poly(4-vinylphenol) (PVPh) and poly(methyl methacrylate) (PMMA) blends has been investigated by FTIR and FT-Raman spectroscopy. Although both techniques provide evidence for the formation of hydrogen bonds between the hydroxyl group of PVPh and the ester group of PMMA, the blends show only a limited degree of hydrogen-bonding interactions when cast from tetrahydrofuran solutions. Comparison of the blends with those cast from a ketone solution and changes in the hydrogen-bonded fractions after thermal cycles indicate that the blends are partially miscible mixtures, in agreement with a previous NMR observation. During the thermally induced evolution of the miscibility, conformational transitions of side chain ester groups in the blend systems, analogous to the phenomenon occurring in PMMA homopolymers, were observed. With prolonged heating, the blends gradually approach the complete miscibility predicted by an association model. It may be concluded that even though the hydrogen bond between PVPh and PMMA favors their miscibility, phase separation still occurs due to the great difference in the two solvent-polymer interaction parameters, resulting in a nonequilibrium multiphase system that is effectively frozen.