A new thermodynamic description of the binary (Cu + Si) system is proposed based on available experimental data. According to the newest information, the phases d and c were treated as phases with a polymorphic transformation at T = 1008 K. The polymorphic transformation of the g phase was also modelled. The liquid phase was described by the associated solution model. Besides optimization of the phase diagram and thermodynamic function, the atomic mobility in a face centred cubic lattice was assessed. Good agreement between calculations and experimental results was found. (C) 2012 Elsevier Ltd. All rights reserved.