Heat of mixing is introduced as a guide for phase stability predictions of polymer mixtures, and an appropriate equation is presented for it. The form of this equation is a combined function of temperature and mixture composition. The capability of the presented equation has been treated qualitatively and it has been shown that all types of exothermic, endothermic, and s-shaped or sigmoidal heat of mixing curves can be produced, Utilizing the low molecular weight analogue calorimetry method, heat of mixing was measured at two temperatures, 27 degreesC and 37 degreesC for three polymer mixtures-poly(styrene)/poly(vinylchloride) (PS/PVC), poly(styrene)/poly(methylmethacrylate) (PS/PMMA), and poly(styrene) /poly(vinylacetate) (PS/PVAc) at an entire composition range. It has been shown that excellent agreement between the results of the calculations and the experimental heat of mixing data was achieved. Using the results of analogue calorimetric measurements for phase stability studies of polymer mixtures, it was found that often, acceptable predictions can be made by this method, but they are not always completely true.