It is proposed that melt-extrusion of poly(3-hydroxybutyrate-co-3hydroxyvalerate) at varying processing temperatures and screw speeds causes changes in the atomic positioning of the oxygen atoms in the helix structure due to torsion of the main chain bonds. By wide angle X-ray scattering it was observed that an increase of the processing temperature has a pronounced e β ect on the intensity of the two sharp Bragg reΓections ( ) and ( ). The unit cell dimensions remain, however, unchanged. The observed repositioning did not change during conditioning for two weeks at 60Β‘C, although the degree of crystallinity increased by approximately 10%. The degree of crystallinity is largely una β ected by the processing conditions. Model di β erential scanning calorimetry experiments indicate that the observed shoulders or double melting peaks with di β erent relative size are due to recrystallization phenomena governed by morphology changes induced by the processing. This study has served as a test case for a Γexible dialogue computer program, designed to be used as a routine tool for determination of the degree of mass crystallinity of polymers from X-ray di β raction data. The change in atomic position of oxygen, as indicated in this work, may be one explanation for the varying enzymatic degradability of the polymers from renewable resources.