A study has been carried out of the low-pressure photochemical vapour deposition of gallium thin films from a gallium triiodide vapour stream, as a function of temperature, precursor partial pressure and illumination intensity. Deposition was found to take place through the photodissociation of adsorbed Gal molecules derived from the thermal decomposition of the triiodide. The reaction system was essentially oxygen-free, as shown by negligible dark deposition rates over the experimental temperature range of 673-873 K. The deposition rate was modelled using thermodynamic data in conjunction with the deBoer and B.E.T. adsorption theories. The resulting gallium films, which oxidized ex situ on exposure to the atmosphere, were characterised by ellipsometry, profilometry measurements of thickness, and EDX analysis for film composition. The possible growth of GaAs by this method was investigated by introducing molecular arsenic or arsenic triiodide into the system. The negligible incorporation of arsenic into the films, even at high arsenic partial pressures, is consistent with the presence of a stable Gal monolayer covering the gallium surface: this layer is taken to inhibit the formation of GaAs. Film growth was found to be consistent with photodeposition from a submonolayer of Gal adsorbed onto the iodine monolayer covering the gallium surface. The deposition of other Group IIIb metals by this method should be practical.