By focusing on a range of fractions of medium-chain-length polyhydroxyalkanoates (mcl-PHA), a more in-depth understanding of the thermal properties and the crystalline structure has been gained. Dual-mode melting was observed for all the fractionated samples. Further DSC analysis revealed that such phenomenon was due to the presence of two crystalline phases (phase I and phase II), forming at different temperatures. As noted, for the samples with higher 3-hydroxytetradecanoate (HTD) comonomer composition, the crystallization rate of phase II was much depressed, which made phase I dominant. The WAXD and MR results indicate that the two phases differ in the crystalline structure. The in situ IR and WAXD registrations indicate that phase I would transform into phase II through a melting-recrystallization process during heating. The high similarity of IR spectra in the range of 1500-900 cm(-1) between phases I and II excludes the possibility of adopting different conformation. It is suggested that major differences between the two crystalline phases lie in the molecular packing. (C) 2009 Elsevier Ltd. All rights reserved.