Abstract: The dependence of the uniqueness in simultaneous determination of substrate optical parameters and film parameters by multiple angle of incidence ellipsometry on the correlation among these unknown parameters was discussed. A general guide to the selection of angle of incidence was proposed. The first derivatives of △ and ψ with respect to n_s, k_s, n_f, k_f, d and angle of incidence φ₀ were computed for Si-SiO₂ and stainless steel-passive film systems. The correlation among the unknown parameters was examined. A random simplex method was developed to obtain results for these two systems.