The defect structure of CdTe doped with Yb and co-doped with Ge was investigated by a set of optical (photoluminescence, absorption, photoconductivity), galvanomagnetic and thermoelectric methods. The results can be explained by a model, in which Yb acts as a deep donor with the energy level at E-V +0.3eV corresponding to the Yb2+/Yb3+ electronic transition. Introduction of Yb with concentration 10(19)cm(-3) in the melt results in a decrease of electrically and optically active acceptor defects in the as-grown crystals and causes a decrease of electrical resistivity of CdTe:Ge. A line at 1.585 eV related to an exciton bound to a complex of Yb and Cd vacancy was observed both in photoluminescence and photoconductivity spectra. (c) 2005 Elsevier B.V. All rights reserved.