The solubility of potassium nitrate (KNO3) in water + ethanol (H2O + EtOH) mixed solvents was measured experimentally in the temperature range of 293.15 to 333.15 K and in the presence or absence of hydrochloric acid (0 <= HCl <= 4 mol kg(solv)(-1)). The significance of these data is directly related to the simultaneous synthesis of KNO3 and HCI using a reactive crystallization process. Ethanol presented a solvent -out effect on KNO3 solubility in water. The presence of HCl in the solution also reduced its solubility. The extended UNIQUAC thermodynamic model was used to correlate the new solid -liquid phase equilibria data for the ternary (KNO3-H2O-EtOH) and quaternary (KNO3-H2O-EtOH-HCl) systems. A new set of interaction parameter values was found that allowed us to predict with a mean absolute error lower than 0.0430 mol kg(solv)(-1).