A complete critical evaluation of all available phase diagram and thermodynamic data has been performed for all condensed phases and relevant gaseous species of the (NaF + AlF3 + CaF2 + BeF2 + Al2O3 + BeO) system, and optimized model parameters have been found. The (NaF + AlF3 + CaF2 + Al2O3) subsystem, which is the base electrolyte used for the electro-reduction of alumina in Hall-Heroult cells, has been critically evaluated in a previous article. The Modified Quasichemical Model in the Quadruplet Approximation for short-range ordering was used for the molten salt phase. The thermodynamic database developed is a first step towards a quantitative study of the beryllium mass balance in an electrolysis cell. In particular, the predominant Be-containing species in the gas phase evolved at the anode were identified; and, for a given beryllium content of the alumina, the beryllium content of the electrolytic bath at steady state was assessed under several approximations. (C) 2012 Elsevier Ltd. All rights reserved.