In situ ellipsometric characterizations of Si1-xGex alloys are presented. The samples are grown by chemical beam epitaxy using silane and germane. The growth process (kinetics, composition etc.) is in situ controlled using a rotating-polarizer multiwavelength ellipsometer. Several examples of heteroepitaxy followed by ellipsometry are shown : desorption of the native oxide and the initial growth stages are analysed at different photon energies. The sequential heteroepitaxy of Si0.85Ge0.15 on Si and Si on Si0.85Ge0.15 is investigated.