The dynamics of particle size distribution (PSD) in a precipitation process are represented by a population balance (PB) and related differential-algebraic equations. The control of PSD is studied by using a closed-form solution of the PB. Batch-to-batch control and on-line single batch control strategies are investigated for controlling a semi-batch reactor. A systematic integration of the two strategies is shown to have a complementary effect on the control performance. (C) 2002 Elsevier Science Ltd. All rights reserved.