Emulsion polymers with well-defined molecular-weight distributions (MWD) were obtained by using a control strategy based on on-line measurements of both unreacted monomer and chain-transfer agent (CTA). The control strategy includes a nonlinear model-based controller that calculates the feed rates of monomer and CTA needed to obtain the desired MWD. This requires a mathematical model for the MWD that was developed based on independent measurements. The control strategy was assessed by computer simulation and experimentally verified by producing polymers with widely different MWDs in the styrene emulsion polymerization using CCl4, as CTA.