In this work the depolarisation phenomena at x-ray diffraction in mosaic crystals were investigated. For this purpose a complete polarisation optical bench including an analyser and two quarter wave plates for the x-ray spectrum was constructed. This bench was used for the investigation of the polarisation properties of the beam diffracted inside a mosaic crystal. Therefore it was assembled at the synchrotron radiation laboratory in Hamburg (HASYLAB). At first the investigation was dominated by the question, whether there is a change of the polarisation state in dependence on the degree of perfection of the used crystal. For this reason the crystal was applied both as-grown and additionally bent. A dependence of the polarisation state on the degree of perfection in the two beam case couldn't be verified. However, a significant change of the polarisation state of the primaryly diffracted beam at the excitation of more than one reflection at once was determined. A systematic investigation of the polarisation state of this beam in dependence on the excitation in many beam geometries was carried out. The primaryly diffracted beam depolarises in these geometries. The phase relation between the two polarisation components, which are perpendicular to each other, gets lost. The strength of the depolarisation is determined by the geometry of the contributing diffraction planes. In this work a model was developed, which explains the experimental results. This model of the gradual intensity transfer is based on an incoherent diffraction process inside the mosaic crystal.