In-situ spectroscopic ellipsometry / CdSe film / Electrodeposition / Optical constantsCdSe films grown on Ti-coated polished steel by constant-current deposition from an aqueous solution containing selenosulfite and cadmium nitrilotriacetate have been studied by spectroscopic ellipsometry under in-situ conditions. Ellipsometric parameters, tanΨ and cos∆, were measured both as a function of deposition time at λ ϭ 825 nm and of wavelength (λ ϭ 500Ϫ900 nm) for constant film thickness from 0.02 µm to 2.3 µm. The model used for data analysis, Ti-substrate | TiO 2 | CdSe-film | solution, with optically homogeneous phases was valid for film thickness larger than 0.3 µm. Refractive indices and extinction coefficients of the 2.3-µm film were derived for λ ϭ 500Ϫ900 nm. A direct band-gap of 1.72 eV was determined. The faradaic efficiency of electrodeposition was 0.93 for a current density of 50 µA cm Ϫ2 .