In this work the nonlinear behaviour of a CSTR in which two exothermic irreversible first-order reactions in series A --> B --> C take place, when the jacket energy balance is incorporated in the CSTR mathematical model, is analyzed. Catastrophe theory is used to study the parameters with respect to which output multiplicities, input multiplicities, isola formation and disjoint bifurcations may exist. One main objective of this work is to address the effect of including the jacket energy balance on the nonlinear behaviour of the CSTR. The results show that the steady-state open-loop nonlinear behaviour of CSTRs, modeled without and with the jacket energy balance, is quite different. Using the software XPP-AUTO different bifurcation maps were obtained. Five operating regions were characterized. (C) 2000 Elsevier Science S.A. All rights reserved.