The initial stages of growth in chemical vapour deposition (CVD) of W from WF6 have been studied in an ultra-high vacuum system using CVD TiSi2 films as substrates. The influence of different surface conditions on the substrate-reduction process was investigated employing three different substrates : (i) oxide-free TiSi2; (ii) TiSi2 covered with a 10 Angstrom SiO2 film and (iii) TiSi2 covered with a mixture of SiO2 acid TiO2. The very low total pressures of WF6 (3 x 10(-4) Pa) used in the experiments reduced the rate of the substrate-reduction step and made it possible to follow the initial growth stages in detail by X-ray photoelectron spectroscopy (XPS). It was found that the oxide-free TiSi2 substrate was highly reactive towards WF6. Significant amounts of TiF3 were formed on the surface during the nucleation stage and incorporated at the W/TiSi2 interface. TiSi2 with a SiO2 film was, as expected, less reactive than an oxide-free silicide substrate. No indication of tungsten oxide formation could be observed in the XPS analysis. W growth occurred by penetration of the oxide him and reduction of WF6 by the underlaying silicide. A further reduction of reactivity was observed using a surface consisting of a mixed oxide of TiO2 and SiO2 prepared by storing the TiSi2 films in humid air. An interaction between W and O was observed on the mixed oxide surface. This was attributed to the presence of hydroxyl groups on the surface.