Biodegradable materials have various important applications in the biomedical field. Poly(hydroxyalkanoate)s are a family of microbially synthesized polymers and known to degrade in vitro and vivo, as well as in microorganism. The structure, crystallization and melting of three types of poly(hyroxyalkanoate)s namely poly(3-hydroxybutyrate) homopolymer, P(3HB), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer, P(3HB-co-3HV, and poly(3-hydroxybutyrate-co-4-hydroxybutyrate) copolymer, P(3HB-co-4HB) were investigated using Fourier transform infrared (FTIR) spectroscopy. P(3HB-co-3HV) and P(3HB-co-4HB) were studied in the presence of different molar ratio of 3HV and 4HB, respectively. The FTIR spectra of samples in solution and as cast film were compared at room temperature, in the region of 4000-400 cm(-1) which showed significant differences between the two phases. Furthermore, the temperature dependent variations of these polymers were also studied between 25 and 200 degrees C. The heating and cooling FTIR experiments of the samples showed that P(3HB-co-3HV) and P(3HB-co-4HB) are reversible while P(3HB) is irreversible. The absorbance variances of the crystalline and the amorphous bands revealed that the structure, crystallinity and thermal behavior of, P(3HB), P(3HB-co-4HB) and P(3HB-co-3HV) are different from each other. This study also allowed us the identification of the bands which are sensitive to crystallinity. For example, the bands at 1185, 1278 and 1724 cm(-1) were found to be sensitive to crystallinity. In the present study, P(3HB) and P(3HB-co-3HV) samples which were modified by applying oxygen plasma to create hydrophilic surfaces were also analyzed. The shifting in the frequency of some infrared bands (i.e. C-O and C-O-C) and the appearance of some bands may indicate some modification on the structure of the polymers. (c) 2005 Elsevier B.V. All rights reserved.