Oxygen ion-assisted TiO2 thin films have been studied by il I situ visible spectroscopic ellipsometry (SE) and transmission electron microscopy (TEM). Influence of the substrate nature and the substrate temperature, the ion kinetic energy E-c and the ion/molecule ratio Phi(i)/Phi(at) was investigated on the microstructure and the optical properties of the films. It is revealed that the refractive index n varies as a function of the average energy per TiO2 molecule, E-d = E-c(Phi(i)/Phi(at)). Conditions for obtaining dense films with a high refractive index (n similar to 2.60 at lambda = 0.45 mu m) and a low extinction coefficient k have been found for E-d > E-dth (E(dth)similar to 50 eV). These dense films are insensitive to moisture adsorption with a low surface roughness. Cross-sectional TEM has been mainly used for microstructure observation and phase identification of the films prepared under different evaporation conditions. Comparison is done in relationship with the optical property measurements.