This paper presents a systematic procedure for multi-unit control design. This procedure results in a decentralised controller for controlling the whole process in which its performance is close to that of a centralised controller. By following this procedure, a multi-unit processing plant is first decomposed into subsystems of smaller dimension, and then the effect of interactions resulting from alternative plant decompositions is analysed to determine the best plant decomposition. For the best plant decomposition, the controller is independently designed for each subsystem The application of this procedure to a reactor/separation process indicates that the performance of this control strategy depends on the manner of decomposing the multiunit processing plant, and that the performance specification is important and must be posed realistically.