In the absorption-evaporative cooling refrigerator, the evaporator works with liquid butane and a gaseous ammonia-butane mixture. In order to design such a refrigerator the mixture behavior has to be known. In this paper such behavior is determined first considering an ideal gases mixture and no ammonia in the liquid butane. For this purpose a psychrometric chart for a pressure of 105 kPa is obtained. To validate these assumptions a second model is chosen that considers a certain amount of non-ideality. The selected model is represented by the equations of Peng-Robinson. The results obtained for the equilibrium conditions are compared with the 100% relative humidity of the psychrometric chart. A satisfactory agreement is found.