There is an increasing interest in the control of particulate systems, with a focus on the control of distributions (e.g. particle size distribution (PSD)). It is known that such distributions have strong correlations with end-product quality indices, this motivating their tight regulation. This paper addresses the design of a control strategy for control of PSD in a semi-batch emulsion polymerization process. In particular, this work presents a hybrid modeling approach for batch-to-batch optimization in which a fundamental population balance model describing PSD evolution is augmented by a Partial Least Squares (PLS) model. This hybrid model is then embedded in a successive quadratic program (SQP) to design surfactant and initiator input trajectories that drive the process to a target PSD. (C) 2003 Elsevier Science Ltd. All rights reserved.