In the present work, a new strategy for the on-line determination of information concerning a free radical polymerisation (overall and individual monomer conversions, and the product of the number of radicals per particle multiplied by the number of particles) is presented. It is based on the construction of a non-linear observer that uses information obtained via a new adaptive inferential calorimetric measurement scheme. The results, presented for the case of the emulsion copolymerisation of methyl methacrylate (MMA) and vinyl acetate (VAc), show that with only a rough model of the physical process, it is possible to obtain reliable estimates of the evolution of the polymerisation reaction.