Amorphous iron(III) oxide thin films have been prepared by vacuum ultraviolet (VUV) photochemically initiated oxidative conversion of [Fe((OBu)-Bu-t)(3)](2) at ambient temperature and pressure. The decomposition process and kinetics for conversion of [Fe((OBu)-Bu-t)(3)](2) precursor film thicknesses of 109-330 nm to much thinner and crack-free iron(III) oxide films (38-54 nm) were investigated. Due to the film shrinkage, higher densities as compared to hydrolysis (3.15 vs. 2.6 g cm(-3)) could be achieved. Mechanistic insight by DFT calculations and FTIR spectroscopy suggests the formation of intermediate Fe(CO2)-ring structures throughout the photochemical conversion process. Resulting iron(III) oxide layers were almost carbon free and exhibiting high transparency in the visible range. The proposed photochemical synthesis pathway represents an attractive route in achieving a dense metal oxide coating at low temperatures, e.g. for coating onto thermally sensitive substrates.