Much research effort has been devoted to the development of sensors capable of providing reliable on-line information about reaction advancement in polymerization processes. In particular, calorimetric techniques which have been widely investigated in the past, still present real difficulties in multipurpose applications. This article describes a new strategy for the accurate determination of conversion during batch polymerizations. In addition to the usual calorimetric data, infrequently-available gravimetric measurements are used to track variations of key parameters, such as the overall heat transfer coefficient. It is shown that accurate estimation of conversion can be obtained through the design of an adaptive state-observer, even if unpredictable conversion-dependent and/or batch-to-batch variations of the system an encountered. Batch and semi-batch experiments were performed in a 7-litre bench-scale well-mixed reactor to evaluate the technique, The method has been successfully applied both to solution polymerization system, i.e. vinyl acetate/butyl acrylate in ethyl acetate, and to emulsion copolymerization operations, i.e. styrene/butyl acrylate in water.