Spectroscopic ellipsometry is a non-invasive tool to characterize thin films. The experimental data are highly accurate and the analysis of the change in polarization upon reflection from the sample surface [1] can lead to determination of any material parameter (e.g. chemical composition, crystallinity, porosity, free carrier concentration), almost all being responsible for the change in optical properties. Performing spectroscopic ellipsometry at different samples temperatures it is possible to determine phase change transitions [2,3]. The advantage of this technique is that small amount of material can be tested, the mass of sample which is meant to be characterized being a limitation of other thermal analysis equipment. Phase change materials [4] (e.g. GeTe, GaSb) are ‘perfect’ materials to prove the usefulness of spectroscopic ellipsometry since the difference between the refractive indices of crystalline phase and amorphous phase are very high [5]. In this work, we will present and discuss the temperature dependence of the optical properties of phase change materials.