A method allowing for the separation of scattering-caused depolarization from depolarization due to film thickness inhomogeneity on isotropic samples is presented. Mueller-matrix analysis shows that M-22 matrix-element is correlated with the scattering-caused depolarization, and correction of the measured depolarization spectra with M-22 element excludes this type of depolarization from the spectra. Ellipsometric measurements performed on polycrystalline ZnO samples with smooth and scattering surface, with uniform and non-uniform layer thickness in every possible combination confirm the suggested method. Besides depolarization, ellipsometric analysis and optical properties of the ZnO films are discussed. (C) 2014 Elsevier B.V. All rights reserved.