An experimental unit has been assembled to carry out on-line optimizing control of the bulk polymerization of methyl methacrylate (MMA). A rheometer-reactor assembly is used. Temperature and viscosity measurements are used to describe the state of the system. The polymerization is carried out under an off-line computed optimal temperature history, T-op(t). A planned disturbance (heating system failure) is introduced at time t(1) This disturbance leads to a fall in the temperature of the reaction mass. A new optimal temperature history, T-reop(t), is re-computed on-line and is implemented on the reaction mass at time t(2) when the heating is resumed. This procedure helps 'save the batch'. A genetic algorithm is used to compute this reoptimized temperature history in a short period of similar to2 min of real time. The feasibility of the on-line optimizing control scheme has been demonstrated experimentally. Replicable results for the viscosity history, eta(t), of the polymerizing mass under several non-isothermal conditions have been obtained. These experimental results are quite trustworthy, even though the model predictions are only in approximate agreement with them, perhaps because of the extreme sensitivity of results to the values of the model parameters.