We have studied the thermodynamic properties of the ammonium sulfate/malonic acid/water system using differential scanning calorimetry and infrared spectroscopy of thin films. Using the results from our experiments and literature data, we have created a solid/liquid phase diagram of the ammonium sulfate/malonic acid/water system for temperatures below 300 K. We also compare our results to the predictions of the aerosol inorganics model (AIM).