A simple model was written to simulate a first-order exothermic reaction taking place in a diabatic controlled-cycled stirred tank reactor ( CCTR) in which the reactor jacket heat capacity is not infinitely larger than the reacting fluid heat capacity. The simulations have shown tpthat including the reactor heat capacity in the model tends to stabilize oscillatory states to a unique steady state. A further result is that a smaller reactor heat capacity results in a shorter transition time to the final steady state. The study shows the possibility that reactor stability and short start-up times may be achieved by simple changes in the reactor design rather than through complex control systems.