Bioreactor control has become very important in recent years due to the difficulty in controlling the highly non-linear behaviour associated with such systems. Model predictive control (MPC) was used to control a non-linear continuous stirred tank bioreactor to an unstable steady state, which was the desired set point. The effect of varying predictor horizon, an important MPC controller tuning parameter, was studied. As the predictor horizon increased, the trajectory moved away from steady state towards a period attractor. The relationship between the changing manipulated input and the system behaviour for different initial conditions as a result of the non-linearities associated with the bioreactor dynamics was also analyzed. The results indicated that the controlled bioreactor can exhibit non-intuitive non-linear behaviour and even a well-designed control scheme may result in very poor performance. (c) 2005 Elsevier Ltd. All rights reserved.