The growth behavior and microstructure of Co-Ge/GaAs films prepared by the high-temperature sequential deposition (HTSD) method have been investigated using X-ray diffraction, cross-sectional transmission electron microscopy, X-ray photoelectron sepectroscopy and atomic force microscopy. Three germanides were formed in the films at temperatures ranging from 300 to 600 degreesC, Polycrystalline CoGe with poor crystallinity was formed at 300 degreesC, On the other hand, epitaxial Co5Ge7 were formed at 400 and 500 degreesC, The epitaxial orientations for the Co5Ge7 films formed at 400 and 500 degreesC are [0 0 1] (1 0 0) Co(5)Ge(7)parallel to [1 1 0] (0 0 1) Ge parallel to [1 1 0] (0 0 1) GaAs and [1 0 0] (0 0 1) Co(5)Ce(7)parallel to [1 1 0] (0 0 1) Ge parallel to [1 1 0] (001) GaAs, respectively. Then, at 600 degreesC polycrystalline CoGe2 was formed in the film. Based on the experimental results, the solid-state reaction mechanism for Co-Ge binary system and the epitaxial growth mode for Co5Ge7 are discussed.