This study examines the isothermal treatment of poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) powders and films. The PHB and PHBV crystallinities were determined using X-ray diffractometry, and shown to increase with temperature (130-150 degrees C) and then decreased from 55% to 45% at 180 degrees C. The crystal morphology of crystal planes (101) and (111) became sharp at a high temperature. The weight average molecular weight (M-w) of PHB decreased from 1,028,000 to 41,800 g/mol when heated at 180 degrees C for 30 min. The molecular weight of PHB decreased more rapidly than that of PHBV with time. No peak signal was observed in gel permeation chromatography after heating at 150 degrees C because the solubility of PHB changed with crystallinity. The thermal behaviors of PHB and PHBV were analyzed by differential scanning calorimetry and thermogravimetric analysis. The roughness, contact angle, and surface morphology of PHB and PHBV films were also measured to determine the surface properties. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3659-3667, 2013