ABSTRACT Studies on discrete control continuous stirred tank reactor (CSTR) with Cholette’s model have not been published in the literature because the mathematical model is far too complicated. This work uses Cholette’s model with discrete proportional control with the mixing parameter n (the fraction of the feed entering the perfect mixing zone) and m (the fraction of the total volume of the reactor) to study exothermic CSTR behavior. Meanwhile, exothermic CSTR continuous proportional control is used to compare the discrete proportional control results. It is found that the nonideal mixing parameters m and n indeed affect the control stability of both the continuous and discrete proportional control. The contribution of this paper is that this analysis can determine the stability region in the control system phase plane. Furthermore, if the nonideal mixing effect is ignored, the stable control system may become unstable and unsafe. Our results are of significant help for CSTR control in industry because the ideal mixing is hardly attainable in reality.