Epitaxial Fe films with excellent properties have been grown on GaAs(100), by the thermal decomposition of Fe(CO)5 at heated substrates. Characterization of the films by LEED and AES at different intervals during deposition, has contributed to an understanding of the growth process. The approach used, since it provides in situ determination of film crystallinity and chemical composition, should be particularly useful in the investigation of the deposition of materials not previously grown epitaxially by MOCVD.