Synchrotron X-ray radiation has been used in situ to follow the crystallization of a hydroxybutyrate oligomer containing 32 repeat units from dilute solution in propylene carbonate, and to study chain unfolding during heating in oligomers with 24 and 32 repeat units. A discontinuity in growth rate occurs at 36-37degreesC for the 32-mer: crystals grown below this temperature contain folded chains, which transform during heating through a process of partial dissolution and re-crystallization to form extended chain crystals. Crystals grown above the transition temperature contain extended chains that do not rearrange during heating. A similar change happens in the 24-mer between 35 and 40degreesC. Thermal expansion in the (110) lattice direction (the fold direction) was in the range 5.3 +/- 0.3 x 10(-5) nm degreesC(-1), with an additional discontinuous increase in 0.001 nm accompanying chain unfolding. (C) 2004 Elsevier Ltd. All rights reserved.