The effective complex dielectric function (epsilon) of Cu and O containing CdTe thin films is reported in the spectral range of 0.05 to 6 eV. The films were fabricated by rf sputtering from targets comprised by a mixture of CdTe and CuO powders with nominal Cu and O concentrations in the range of 2-10 at.%. Low concentration levels improved the crystalline quality of the films. Spectroscopic ellipsometry and transmittance measurements were used to determine epsilon. The critical point energies E-1, E-1 + Delta(1), and E-2 of CdTe are red-shifted with the incorporation of Cu and O. Also, an absorption band is developed in the infrared range which is associated with a mixture of CdTe and low resistivity phases Cu2 - xTe according to an effective medium analysis. The elemental distribution of the films was mapped by energy dispersive X-ray spectroscopy using scanning transmission electron microscopy. (C) 2014 Elsevier B.V. All rights reserved.