Polycrystalline copper-indium-(gallium)-sulphide (CI(G)Su) absorbers were analysed by spectroscopic ellipsometry (SE) with special emphasis on the optical band gap energy. Rough CI(G)Su absorber films grown by reactive magnetron sputtering were peeled off from molybdenum coated glass substrates. The smooth back side of CI(G)Su absorbers was suited for the SE analysis. Furthermore, these samples were covered with a thin zinc oxide (ZnO) layer and heat-treated with a commercial xenon flash lamp annealing system (FLA) as well as by thermal annealing in an argon atmosphere. The effect of zinc on CI(G)Su absorber films was studied by secondary ion mass spectrometry depth profiling as well as by SE analysis. The optical modelling of spectral Stokes parameters was performed by using a multilayer approach over a spectral range from 1.5 to 4.3 eV. Spectral absorption coefficients were calculated in every process stage, i.e. (i) peeled samples, (ii) ZnO deposition, (iii) FLA treatment and (iv) etching of the ZnO. Special emphasis was given to the shift of the optical band gap due to the various treatments. While the SE analysis was quite sensitive to the change of optical band gaps due to a varying gallium content in the CI(G)Su absorber layers, a significant shift of the optical band gap due to increasing zinc content was not detectable. (C) 2014 Elsevier B.V. All rights reserved.