A non-linear state estimator that provides on-line information on residual monomer concentration and radical concentration is experimentally validated under dynamic conditions, and in particular in the presence of secondary particle nucleation. The proposed estimator uses the heat of reaction as an input. Practical methods for determining the initial values of the parameters in the energy balance are also proposed and tested on laboratory and pilot scale installations. It is shown that this method provides a robust tool for the on-line monitoring of batch (with and without shot additions of monomer, surfactant and initiator) and semi-batch emulsion polymerisation reactors. The observer was shown to converge rapidly, even under conditions where the number of particles per litre of emulsion (N-P) changed rapidly.