The present study aims at bringing out the best features of model-based control, when applied to highly non-linear systems like pH-controlled fed-batch processes. For these processes, control of pH by conventional PID controller fails to provide satisfactory performance, because of the extreme non-linearity in the pH dynamics. In the present study, for a fed-batch neutralisation process, a non-linear control law has been derived for the model-based PI controller. Typical problems in process control like sampling, delay and perturbations in model parameters are addressed in this study using model-based control. The simulation results show the superior performance and robustness of the model-based PI controller over that of the conventional PI controller.