Thermodynamic modelling of the Mg-Cu-Y system is carried out as a part of thermodynamic database construction for Mg alloys. This system is being modelled for the first time using the modified quasi-chemical model which considers the presence of short range ordering in the liquid. A self-consistent thermodynamic database for the Mg-Cu-Y system was constructed by combining the thermodynamic descriptions of the constituent binaries, Mg-Cu, Cu-Y, and Mg-Y. All the three binaries have been reoptimized based on the experimental phase equilibrium and thermodynamic data available in the literature. The constructed database is used to calculate and predict thermodynamic properties, the binary phase diagrams and liquidus projections of the ternary Mg-Cu-Y system. The current calculation results are in good agreement with the experimental data reported in the literature. (C) 2008 Elsevier Ltd. All rights reserved.