Strontium-bismuth-tantalate (SBT) is a new kind of dielectric layer material for use in semiconductor devices. The optical layer parameters of SBT were characterized by spectroscopic ellipsometry using the well-known Cauchy model as well as the Adachi model (Phys. Rev. B 35 (1987) 7454-7463). A comparison of both models was performed. Furthermore, these optical data were compared with the physical and chemical behavior examined by Rutherford backscattering (RBS) and X-ray diffraction (XRD). As a result, it was possible to fit the measured spectra with both optical models. But with the Adachi model, it was possible to evaluate the optical layer parameters in a wider range than in the measured spectral range covering the region of the band gap. The Adachi model provides electronic layer parameters like the transition energy E-0 Our investigations also include the determination of the stoichiometry dependence of the optical layer parameters. (C) 2003 Elsevier B.V. All rights reserved.