The thermal behaviour of blends of 4-n-octyloxybenzoic acid with polystyrene, poly(4-vinyl pyridine) and poly(2-vinyl pyridine) has been characterised using differential scanning calorimetry and polarised light microscopy. The acid is essentially immiscible with polystyrene over the whole composition range. However, molecular mixing is observed for blends containing up to an acid mole fraction of ca. 0.3 with either poly(4-vinyl pyridine) or poly(2-vinyl pyridine). This miscibility is driven by hydrogen-bond formation between the acid and pyridine groups. At higher concentrations of the acid, phase separation occurs although the presence of hydrogen bonding is evident in the IR spectra. No pronounced differences are observed between the blends containing the differing isomers of poly(vinyl pyridine). A model is proposed which interprets these data in terms of two competing processes: the self-association of the acid-forming dimers and the specific interaction between the acid and pyridine groups, yielding hydrogen-bonded complexes.