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 kgsolv–1). The significance of these data is directly related to the simultaneous synthesis of KNO3 and HCl 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 kgsolv–1.