Wide angle X-ray experiments evaluated by recent developments of X-ray line profile analysis allow for the detection of the presence of dislocations as well as to determine their density in crystalline materials. The application to semicrystalline polymers not only provides information on the crystal size and the dislocations butin combination with in situ deformationalso information on the evolution of these microstructural parameters. Investigations on cold rolled and on uniaxially compressed samples of a-phase isotactic polypropylene (alpha-iPP) as well as poly(3-hydroxybutyrate) (P3HB) are presented. The synchrotron experiments reveal a dislocation governed deformation process in alpha-iPP center dot P3HB, however, deforms by a process not including dislocation generation. Here, microcracking and strain localization in the amorphous phase seem to be the predominant deformation mechanisms. (C) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012