Aluminum titanate thin films were prepared by ion beam induced chemical vapor deposition (IBICVD). By this method it is possible to control the Al/Ti ratio in the film. Films with Al/Ti similar or equal to 2 (i.e., a ratio similar to Al2TiO5) were characterized by scanning electron microscopy and transmission electron microscopy, x-ray photoelectron spectroscopy, electron energy loss spectroscopy at the electron microscope, and x-ray absorption spectroscopies. These techniques showed that after calcination the films are amorphous, although at T=823 K the local environment around the Ti and Al ions is very similar to that of these ions in a crystalline Al2TiO5 phase. At T greater than or equal to 1023 K the formation of a TiO2 phase shows that the aluminum titanate dissociates at this temperature into TiO2 and Al2O3. Reaction of the latter with the SiO2 substrate at T greater than or equal to 1523 gives a mullite phase. The optical quality of the aluminum titanate films was checked by ultraviolet-vis spectroscopy. After calcination, the aluminum titanate films prepared by IBICVD were very transparent. Those with a ratio Al/Ti=2.18 have a diffraction index n=1.9 and a band gap E(g)=4.1 eV. These values and the absence of the characteristic absorption gap of TiO2 support the formation of a mixed AlxTiyOz phase.