Control of nonlinear systems with input and output multiplicities is theoretically analysed. The system is an isothermal CSTR with a nonideal mixing represented by a Cholette’s model and with a chemical reaction rate k(1)c/(1 + k(2)c)(2). The system exhibits input multiplicities in the feed concentration and also output multiplicities in the reactor exit concentration. For control system design the nonlinear system is represented by a nonlinear gain cascaded with a linear dynamical system. The performance of an integral controller designed for the overall linear system is compared with that of a nonlinear integral controller. The nonlinear integral controller does not have the problem of moving the input to a different level for a square pulse disturbance.