The influence of cooling rates on the thermal behavior and solid-state morphologies of polyhydroxyalkanoates have been investigated. The thermal behavior was studied by differential scanning calorimetry (DSC). The crystal structures (similar to angstrom), lamellar (tens of nanometers), fibrillar (several hundred nanometers), and spherulitic (similar to mu m) morphologies of poly (3-hydroxybutyrate) (PHB) and the copolymers of poly (3-hydroxybutyric acid-co-3-hydroxyvaleric acid) (PHBV) and poly (3-hydroxybutyric acid-co-3-hydroxyhexanoic acid) (PHBHx) crystallized under different cooling rates were studied using simultaneous small angle X-ray scattering (SAXS) and wide angle X-ray scattering, simultaneous ultra small angle X-ray scattering (USAXS) and SAXS, and polarized optical microscopy, respectively. The experimental results showed that the lamellar and spherulitic morphologies depended strongly on cooling rates. However, there was little influence of cooling rates on the crystal structures. (c) 2008 Wiley Periodicals, Inc.