The structure and free volume properties of semi-crystalline poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) were investigated in this study. The structure change and conformational motion of PHBV during melting and crystallization process were discussed by in situ FTIR. The free volume within the amorphous phase and its temperature dependence in the PHBV membrane, which was prepared by the compression molding method with isothermal crystallization processes, were characterized by using positron annihilation lifetime (PAL) spectroscopy. From the lifetime parameters, the temperature dependence of free volume size, amount. distribution, and fractional free volume, and the thermal expansion of free volume and/or polymer were discussed. Furthermore, the knee temperature was first observed in the melting process of the crystallized PHBV membranes. It indicated that there was structural transition of polymer chains during melting as :he corresponding results observed with in Situ FTIR measurement. (c) 2009 Elsevier Ltd. All rights reserved.