Thermal properties of poly(vinyl phenol) (PVPh) and poly(hydroxybutyrate) (PHB) blends were investigated by differential scanning calorimetry over the entire range of composition. Mixtures with both bacterial (tactic) PHB and synthetic (amorphous) PHB were considered. In all cases, blends showed a unique glass transition temperature. The presence of 40% PVPh prevents bacterial PHB crystallization. Infra-red spectra showed the presence of hydrogen-bonded carbonyl signals which increased with PVPh fraction, showing the extent of specific interactions.