Bacterial poly(3-hydroxybutyrate) (PHB) - a new biopolymer and its derivatives are being extensively applied in medicine, biotechnology and membrane technologies. The relationship between structure-transport parameters and molecular mobility of PHB was investigated by quartz McBain＇s microbalance, X-ray diffraction and ESR method. Two types of the PHB membranes with different structural organisation-isotropic and textured-were studied. The specific features of the sorption and diffusion characteristics of PHB are related to the structural organisation. The polymer matrixes are characterised by significantly microstructural heterogeneity as it was shown by the spin probe (TEMPO/TEMPOL) measurements. A non-crystalline phase of PHB includes two kinds of microfields with different molecular mobility. Moisture is found to change both the velocity of molecular motion and the mole content of such microfields. The diffusion behaviour of the membranes has been explained in accordance with dynamic information based on the ESR method.