Phase diagrams for poly(n-hexyl methacryrate) blends with a series of styrene-co-vinylphenol polymers have been mapped using light-scattering techniques. These are compared to theoretical phase diagrams calculated using an association model that incorporates the various interactions in hydrogen-bonded polymer systems. The thermodynamics of these blends is strongly affected by the competition between the self-associating and interassociating hydrogen bonds and the unfavorable dispersive interactions. The balance of these competing factors controls the temperature and composition of phase separation. The results of the theoretical predictions are found to be in good accord with the experimental phase diagrams. They also help in explaining the molecular weight independence and the skewness (asymmetry) of the phase diagrams.