The titania coatings have been deposited on polyethylene terephtalate (PET) by r.f. magnetron sputtering in an oxygen-argon plasma from a pure titanium target. The titania films always consist of a mixture of amorphous and crystallized phases of the anatase structure. The composition (O/Ti atomic ratio) established by Rutherford backscattering spectrometry depends on the deposition parameters and mainly on the oxygen partial pressure. The crystallinity, the compactness, the stresses and the gas barrier effect of the titania coatings increase with the kinetic energy of the sputtered particles. This varies in the same way as the r.f. power and in the opposite manner to the oxygen partial pressure. The refractive index increases all along with the compactness of the titania film; the extinction coefficient decreases and the optical bandgap values increase when the O/Ti atomic ratio increases. Good adhesion of a titania coating on a PET film needs at the same time activation of the surface of the PET by a CO, plasma treatment and optimization of the sputtering parameters.