Effect of impurities (oxygen, carbon) on initial stages of phase formation for the system of Ti overlayer deposited in a 2 x 10(-7) Torr vacuum on Si0.83Ge0.18 layers epitaxially grown on (001) Si has been studied by Auger electron spectroscopy, X-ray powder diffractometry, conventional and high resolution transmission electron microscopy and energy dispersive spectroscopy. The change in impurity contents at the Ti/SiGe interface was realized by changing the thickness of the Ti overlayer. Two sets of Ti/SiGe/Si samples with 10 and 60 nm thick Ti were investigated. It was shown that for the samples with 60 nm thick Ti the content of the oxygen and carbon at the Ti/SiGe interface was one order of magnitude lower than that in the samples with 10 nm thick Ti overlayer. The first crystalline phase formed as a result of annealing at a vacuum better than 2 x 10(-7) Torr was Ti-5(Si,Ge)(3) for samples with a high level of the impurities and C49-Ti(Si,Ge)(2) for samples with a low level of the impurities. An amorphous phase was revealed for both sets of specimens during the initial stages of phase formation. The results obtained were interpreted within the framework of a recently developed model of phase formation.