GexSi1 − x thin films (x = 0.03–0.75) were grown on Si(111), quartz and graphite by LPCVD using SiF2 and GeCl4 as the precursors at 450–700°C. Thin films prepared at 550–700°C contained polycrystalline GexSi1 − x. The composition of the thin film can be controlled by adjusting the reaction temperature and the relative inlet quantity of GeCl4. A linear correlation between the lattice parameters of the prepared GexSi1 − x thin films and the Ge contents x is established. A plausible reaction mechanism is proposed and discussed.