The enthalpy of mixing (at constant temperature and pressure) an aqueous solution of ammonia with an aqueous solution of either sodium sulfate or ammonium sulfate was measured by batch calorimetry at temperatures from 313 to 373 K. The experimental results are, first, used for comparison with predictions from a modified Fitter's model for the solubility of ammonia in aqueous solutions of those strong electrolytes and, secondly, to tune the influence of temperature on some parameters of that model.