A dynamic model has been developed describing the emulsion polymerisation of styrene within a batch/semi-batch stirred tank reactor (BSTR). This model includes the initiation, propagation and termination steps for styrene polymerisation, along with the relevant mass balance equations (including those for polymeric radicals) and energy balance equations-the latter covering heat of reaction, internal and external heat transfer effects, as well as external heat losses. The resulting set of (differential/algebraic) equations was solved for both species concentrations and temperature profiles as functions of time. Experiments were conducted in a laboratory BSTR instrumented with platinum resistance thermal transducers and gravimetric conversion measurement devices. The model predictions compared well with inferential calorimetric measurements which were validated using experimental gravimetric data. Subsequent implementation of a model-based optimal control strategy resulted in a 13% relative increase in monomer conversion, together with a 28% reduction in batch time.