The miscibility and transesterification of poly(hydroxyl ether of bisphenol A) (phenoxy) and poly(3-hydroxybutyrate) (PHB) were investigated by means of differential scanning calorimetry (DSC) and Fourier transform IR spectroscopy (FTi.r.). The blends, prepared by casting from a dimethylformamide solution at 80 degrees C and drying in vacuum at 80 degrees C, heated from room temperature to 180 degrees C at a rate of 10 degrees C min(-1), are miscible at 180 degrees C when the PHB content is less than about 20 wt.% and almost immiscible when the PHB content is over 20 wt.%. The FTi.r. spectra reveal that the PHB content in the THF soluble fraction of the blends containing 30 wt.% PHB annealed at 180 degrees C increases with increasing annealing time. Both the extraction and the FTi.r. experiments indicate that transesterification occurs between phenoxy and PHB. When the transesterification is complete, hence when the annealing time is long enough, a single phase is formed. During transesterification, the number of non-hydrogen-bonded OH groups of phenoxy first increases, passes through a maximum and then decreases with increasing annealing time at 180 degrees C, becoming negligible once a single phase is formed. An explanation is proposed for this behavior.