The solubility of sodium sulfate (Na2SO4) in (water + ethanol) mixed solvents was measured using an analytical gravimetric method. The experiments were performed at 298.15 and 308.15 K, in the absence and presence of hydrochloric acid at low concentrations (in the range of 0.2-1.0 m). The experimental conditions (HCl concentration and temperature) were relevant to a new reactive crystallization process leading to obtain simultaneously Na2SO4 and HCl. Some results for the Na2SO4-(water + ethanol) were compared with experimental data available in the literature confirming the methodology reliability and reproducibility. In comparison with HCl-free system, at 308.15 K, the solubility of Na2SO4 in (water + ethanol = 0.05 wt%) mixed solvent decreases in the concentration range of 0.2 m < HCl < 0.4 m. For higher HCl concentrations, the solubility of Na2SO4 increases. The values of (ethanol-H+ ions) interaction parameters for the extended UNIQUAC thermodynamic model were not reported in the open literature. Thus, their new set was adjusted. The excellent capability of the model, to predict accurately the Na2SO4-(water + ethanol)-HCl solid-liquid phase behavior, is illustrated. (C) 2014 Elsevier B.V. All rights reserved.