Polyhydroxyalkanoates (PHAs) produced from fermented molasses and synthetic feeds containing single volatile fatty acids (VFAs) by an open mixed culture enriched in glycogen accumulating organisms (GA0s) were characterized with regards to molecular weight and thermal properties. The polymer contained five types of monomers, namely 3-hydroxybutyrate, 3-hydroxy-2-methylbutyrate, 3-hydroxyvalerate, 3-hydroxy-2-methylvalerate and 3-hydroxyhexanoate in different ratios depending on the VFA composition of the substrate. Polymers produced from fermented molasses had weight average molecular weights (Mw) in the range (3.5-4.3) x 10(5) g/mol and polydispersity indexes (PDI) of 1.8-2.1 while polymers produced from synthetic VFAs had M, of (4.5-9.0) x 10(5) g/mol and PDI of 1.7-3.9. Thermal properties such as glass transition temperature (-14 degrees C to 4.8 degrees C), melting temperature (89-174 degrees C) and melting enthalpy (0-82.1 J/g) were controlled in broad ranges by the monomer composition. The decomposition temperatures of the polymers produced were between 277.2 degrees C and 294.9 degrees C, and independent of monomer composition and molecular weight. (C) 2010 Elsevier B.V. All rights reserved.