Epitaxial ErSi2-x have been grown by reactive deposition epitaxy technique on [111]Si substrate in a high vacuum system (operating pressure similar to 10(-8) Torr) at different substrate temperature. The residual gas pressure during deposition was controlled by a mass spectrometer and the O partial pressure was varied in the range 1 X 10(-9)-4 X 10(-8) Torr. Rutherford backscattering spectrometry in combination with channeling effect, nuclear reaction analysis O-16(d,p)O-17 and transmission electron microscopy have been used to independently determine the O and Er concentration and thr crystalline quality of the layer, The correlation between the O concentration in the silicide and the deposition parameter has been determined. Good quality epitaxial layers were been obtained only if O was incorporated in the silicide at a concentration of about 7 arm.