Thermally safe operation of a SBR implies that conditions leading to a large accumulation of unconverted reactants are avoided. In this work a new method for assessing the thermal stability of semibatch processes is presented. The thermal runaway region is characterised as a function of the design parameters using a criterion based on the Lyapunov exponents. The results are in good agreement with existing methods. The advantages of this method are its simplicity and flexibility since it can be applied to complex systems without previous particular definitions.