Iron oxide thin films have been prepared at room temperature by ion beam induced chemical vapour deposition. The films were grown by bombardment of a substrate with O ϩ 2 or O ϩ 2 ϩAr ϩ mixtures, while a Fe(CO) 5 volatile precursor was dosed on the surface. In both cases, the films were dense and had a low roughness. After preparation, the films prepared with O ϩ 2 ions consisted of hematite (α-Fe 2 O 3 ), although with a low degree of order as revealed by grazing angle X-ray diffraction (GXRD), transmission infrared (IR) spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy. A progress in the crystallisation process of these films occurred by annealing at 573 and 773 K. In all cases, the crystallographic c axis of the hematite structure was oriented perpendicular to the film surface. Samples prepared with the O ϩ 2 ϩAr ϩ mixture were amorphous or badly ordered. Rutherford backscattering spectroscopy (RBS) showed that in these films Ar (ෂ4% atomic) became homogeneously incorporated within the Fe 2 O 3 lattice. These Ar atoms remained incorporated even after annealing at 773 K. At this temperature, a sudden crystallisation of α-Fe 2 O 3 occurred, resulting in the same preferential orientation of the c axis as in the samples prepared in absence of Ar. The extinction of some GXRD peaks for these annealed samples is correlated with the presence of Ar atoms within the α-Fe 2 O 3 lattice. The possibility of controlling the structure of thin films prepared by ion beam assisted methods by incorporation of Ar into their structure is discussed.