A nonlinear geometric control is used to control the concentration of the monomer in solution polymerization processes. The concentration of the monomer is an important parameter in terms of the final product quality, process productivity, and security. The process is monitored using a near-infrared spectrometer that was calibrated using the partial least-squares optimization technique to estimate the concentration of the monomer in the reactor. The control strategy makes use of a high gain observer to estimate the reaction rate necessary in the control loop. The controller was implemented on an industrial pilot reactor during the solution polymerization of acrylic acid.