A decoupled control law is proposed to control the concentration of monomer in the polymer particles in emulsion homopolymerization, copolymerization and terpolymerization processes and simultaneously to insure the production of a homogeneous polymer composition. Nonlinear geometric controllers are used to calculate the monomer flow rates that accomplish this purpose. The controllers are based on the estimation of the residual number of moles of free monomer that is obtained by calorimetry and using nonlinear high gain observers in this work. The technique is experimentally validated using the butyl acrylate, methyl methacrylate and vinyl acetate monomers. (C) 2003 Elsevier B.V. All rights reserved.