Reaction calorimetery was used to monitor and control unseeded semi-batch emulsion copolymerization of VAc and BuA. An external PC, using the information derived from a commercial reaction calorimeter, calculated on-line the thermal and material balances of the calorimetric reactor, and thus the monomer conversions. For a desired copolymer composition profile, previously calculated conversion-dependent monomer feed profiles were used to determine the rates of monomer addition every 20 seconds. Gas chromatography and gravimetry confirmed that copolymers having the desired variable composition profiles were successfully obtained. The control strategy devised lends itself to polymer quality control and safe operation, since variable monomer inhibitor levels or sudden inhibition during the reaction do not lead either to the formation of off-composition polymer nor to monomer accumulation in the reactor. Furthermore, the rate of polymerization can be reduced by fixing upper limits for monomer concentrations in the polymer particles, useful in cases where the minimum-time optimal feed profile, which corresponds to maximum swelling, might lead to thermal runaway.