AbstractThe ellipsometric spectra of CuIn3Se5, CuGa3Se5 and CuGa5Se8 ordered vacancy compounds (OVC) have been studied in the spectral range from 0.8 to 4.4 eV. The structures observed in the measured spectra were analyzed by fitting the second derivative of the experimental spectrum to analytical line shapes. The energies corresponding to different electronic transitions were determined. The lowest direct gap (E 0) is characterized by 3‐dimensional critical points, while higher energy structures are characterized by 2D critical points. Optical constants related to dielectric function, such as the complex refractive index n *(ω ), extinction coefficient k (ω ), absorption coefficient α (ω ), and normal‐incidence reflectivity R (ω ), of studied OVC were determined. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)