Ellipsometry is a powerful and convenient technique that is widely used to determine the thickness and optical characteristics of polymer thin films. The determination is accomplished by modeling the measured change in the polarization of an electromagnetic wave upon interacting with the thin film. However, due to the strong statistical correlations between the fit parameters in the model, simultaneous determination of the thickness and the refractive indices of optically anisotropic ultrathin films using ellipsometry remains a challenge. Here, we propose an approach that can be used to obtain reliable values of both the thickness and the optical anisotropy of ultrathin polymer films. The approach was developed by performing spectroscopic ellipsometry measurements on thin films of hydrophobic polystyrene and hydrophilic chitosan of thickness between a few tens to a few hundred nm and whose absolute value of the birefringence differed by approximately an order of magnitude. Careful consideration of the characteristics of the root mean squared error of the fits obtained by modeling the ellipsometry data and the statistical correlations between the fit parameters formed the basis of the proposed approach.