Monitoring of the particle size distribution (PSD) in emulsion polymerization is a very important task to improve the product quality. However, on-line sensors used to measure PSD, such as capillary hydrodynamic fractionation, usually require taking samples and the analysis time is still too long for control. This work proposes the combination of mathematical models with existing on-line sensors to obtain a continuous estimation of the PSD by constructing nonlinear estimators. Simulations of the observers show that they are able to detect particle nucleation and to give an estimation of the whole PSD. (c) 2005 American Institute of Chemical Engineers.