The thermodynamic modeling and optimization of the Ge-Sb and Ge-Sb-Sn systems were critically carried out by means of the CALPHAD (CALculation of PHAse Diagram) technique. The solution phases, liquid, diamond, bct and rhombohedral, were described by the substitutional solution model. The compound SbSn was treated as the formulae (Ge,Sb,Sn), (Ge,Sb,Sn), in the Ge-Sb-Sn system. A self-consistent thermodynamic description of the Ge-Sb-Sn system was developed. Three isothermal sections at 692, 594 and 518 K, the projection of the liquidus surfaces, and the complete reaction scheme for the Ge-Sb-Sn system in the literature were reproduced. (C) 2011 Elsevier B.V. All rights reserved.