A high-gain nonlinear cascade state estimator is presented in this work and used to monitor conversion and polymer composition in an emulsion copolymerization process:. The advantage of this cascade observer is that it requires neither occasional online samples nor prior. knowledge of the dependence of the overall heat-transfer coefficient with the solids content of latex. A simple test is used to ensure that the results are robust to process noise and to rapid changes in conditions during the semibatch phase. Applications of this. technique are presented in both laboratory-scale and industrial pilot-scale reactors.