Control of nonlinear systems represented by a linear system cascaded with nonlinear gain having two input multiplicities (i.e. gains with opposite sign) is theoretically analyzed. The system is a continuous stirred tank reactor carrying out Van de Vusse reaction and exhibiting input multiplicities in the manipulated variable (u) on the product yield. Conventional PI controller designed for the linear portion of the system followed by the solution of the nonlinear gain part gives a nonlinear controller. The performance of the nonlinear controller is compared with a linear PI controller based on the overall linear system.