The polarization state of the transmitted beam in Bragg-case X-ray dynamical diffraction was determined in dependence on the deviation from the center of the reflection curve. Because of their low absorption lithium fluoride crystals were examined. The transmission rocking curves were detected by means of a linear analyzer set at different azimuthal positions. In this way the effect of the phase plate can be easily visualized from the occurrence of intensity oscillations. An almost circularly polarized beam could be produced from an incident linearly polarized beam using this phase retarder. The complete polarization state of this beam was determined by inserting a second phase retarder adjusted to give a phase shift of  p/2 and calculating the three Stokes parameters.