A thermodynamic database for the Mg-Ce-Mn-Y system has been prepared by critical evaluation of all available phase diagram and thermodynamic data in order to obtain a set of optimized model parameters for the Gibbs energy of all phases as functions of temperature and composition. For the liquid alloys, the modified quasi-chemical model (MQM) in the pair approximation was used to treat short-range ordering. Solid solutions were modeled using the compound energy formalism. Sample calculations are given to illustrate the use of a database in the design of Mg alloys.