The so-called glycogen accumulating organisms (GAOs), found in mixed culture anaerobic-aerobic wastewater treatment processes, are used to produce polyhydroxyalkanoates (PHAs) copolymers from a single, simple carbon source-acetate. These polymers consist of 3-hydroxybutyrate (HB), 3-hydroxyvalerate (HV), 3-hydroxy-2-methylvalerate (HmV) and 3-hydroxy-2-methylbutyrate (HmB) with a non-HB fraction of up to 37%. A detailed characterisation using H-1 and C-13 nuclear magnetic resonance (NMR) spectroscopy demonstrates that these products are true copolymers if produced under anaerobic conditions. The HmV and HmB monomers are identified and their incorporation in the polymer verified via two-dimensional (2D) NMR spectroscopy. The monomer sequence distribution analysis using a known polymer characterisation model suggests that these polyesters are mixtures of random copolymers and thus likely possess desirable properties for practical use. This assessment is supported by the polymers' physical characteristics. It was further revealed from the study that the inclusion of aerobic periods into the process could improve the overall polymer yields but resulted in production of HB monomers only, likely in the form of separate homopolymer blocks. The study for the first time investigated the microstructure of GAO-produced biopolymers; and shows that GAOs have a potential for producing high-quality polymers from a simple carbon source. (C) 2008 Elsevier Ltd. All rights reserved.