Abstract Ce, Sm, Dy, Er and Yb doped PbTiO 3 thin films were deposited by sol–gel method on (111)Pt/Ti/SiO 2/Si substrates. The optical properties of the films were characterized by means of ellipsometry using a HeNe-laser source ( λ=632.8 nm). Real ( n) and imaginary ( k) parts of the refractive index were obtained applying the Fresnel equation. It is shown that for lead titanate thin films doped with 2 mol.% of lanthanide (Ln) ions the real part of the refractive index decreases smoothly with increasing atomic number of the element, with the exception of the Ce doped film. The experimental results are explained taking into account the electronic structure of the dopants. Specific results of Ce are explained by its oxidation state, which has been shown by means of X-ray photoelectron spectroscopy to be +4. Additionally, noticeable correlation of the optical properties of the films with respect to doping level was observed.