State estimation techniques are used to on-line estimate reaction rates in well-mixed semicontinuous reactors. The proposed scheme assumes the availability of on-line measurements of the total mass of the reactants in the reactor, and copes with the lack of knowledge of the reaction rate dependence with concentration. Concepts from system theory are used to reduce the degrees of freedom that the pure Kalman filtering design produces. By the use of three examples of semicontinuous emulsion copolymerization, it is shown that accurate estimates of the reaction rates are possible in the presence of inaccurate measurements of the incoming flow rates and much earlier than the establishment of the pseudo-steady-state condition. The proposed technique is applied to the experimental data collected during the semibatch polymerization of the following ternary systems : (a) acrylonitrile-ethyl acrylate-N-methylacrylamide and (b) butyl acrylate-styrene-acrylic acid.