This article presents and develops the thermodynamic cot-relation model to obtain the solubility of polyhydroxyalkanoates (PHAs) in solvents. This model offers a new way to evaluate the polymer solubility parameters and regressions between experimental solubility and solubility parameters. Important parameters influencing solubility such as temperature, polymer molecular weight, crystallinity, and the kind of PHAs were studied. Experiments were carried out on PHB-co-Hx copolymers of different contents. Finally, a model which takes into account these parameters were established and tested on 232 experimental points. The method developed could be used for the selection of a solvent with high efficiency, low price, and low toxicity. (c) 2007 American Institute of Chemical Engineers.