This paper describes the application of nonlinear model predictive control (NMPC) to the thermal control of a batch or semi-batch chemical reactor fitted with an alternative heating/cooling system. The strategy of the nonlinear control system is based on an open-loop constrained optimisation problem which is solved repeatedly on-line by a sequential optimisation and solution strategy on a PC for a typical temperature sampling rate. The feasibility of such a controller is examined on simulations and experiments on a 16 litres pilot plant. The presented results are satisfactory and are promising for an industrial use of such a technique.