Two sets of ZrO 2 thin films have been prepared at room temperature by ion beam induced chemical vapour deposition and subsequently annealed up to 1323 K. The two sets of samples have been prepared by using either O þ 2 or mixtures of (O þ 2 þ Ar þ ) ions for the decomposition of a volatile metallorganic precursor of zirconium. The structure and microstructure of these two sets of samples have been determined by means of X-ray diffraction, Fourier transform infrared spectroscopy and positron beam analysis (PBA). The samples were very compact and dense and had a very low-surface roughness. After annealing in air at T P 573 K both sets of films were transparent and showed similar refraction indexes.For the (O þ 2 þ Ar þ )-ZrO 2 thin films it is shown by X-ray photoelectron spectroscopy and Rutherford back scattering that a certain amount of incorporated Ar (5-6 at.%) remains incorporated within the oxide lattice. No changes were detected in the amount of incorporated Ar even after annealing at T ¼ 773 K. For higher annealing temperatures (T > 1073 K), the amount of Ar starts to decrease, and at T ¼ 1223 K only residual amounts of Ar (<0.4%) remain within the lattice. It has been found that as far as Ar atoms remain incorporated within the ZrO 2 network, the (O þ 2 -Ar þ )-ZrO 2 films present a cubic/tetragonal phase. When the amount of ''embedded'' Ar decreases, the crystalline phase reverts to monoclinic, the majority phase observed for the (O The incorporation of Ar within the ZrO 2 thin film structure, is proposed as the main factor contributing to the stabilisation of the cubic/tetragonal phase of ZrO 2 at room temperature.