Phase contrast provides new possibilities in X-ray imaging, offering up to 1000 times higher sensitivity than standard absorption contrast. In propagation based phase contrast imaging, a quantitative relationship exists between intensity in the image plane and the phase shift induced by the object. Inversion of this relationship is called phase retrieval. Used as input to a 3D tomographic reconstruction algorithm this gives a reconstruction of the refractive index. Several methods for phase retrieval have been described, but few quantitative studies have been performed. In this paper we describe three phase retrieval methods, respectively based on the transport of intensity equation (TIE), contrast transfer function (CTF) and a mixed approach recently developed at the ESRF. The methods are evaluated using simulated and experimental data in the case of mixed absorption and phase objects. Using the TIE on simulated data we obtain a reconstruction with a mean error of 10%, but fail to achieve a qualitatively acceptable reconstruction of experimental data. The CTF approach yields qualitative reconstructions both using simulated and experimental data. Using the mixed approach, we obtain reconstructions with close correspondence to expected values with an average errors of 3.8% for the simulated and 5.9% for the experimental data