The effect of agitation on the semicontinuous emulsion copolymerization of styrene and butyl acrylate was investigated. Both neat monomer addition and preemulsified feed were used. Experiments with and without a chain-transfer agent were carried out. For neat monomer addition, a mild degree of agitation (Ն 0.1 kW/m 3 ) was required to avoid monomer mass-transfer limitations, but even a moderate degree of agitation (0.3 kW/m 3 ) was not enough to overcome the CTA mass-transfer limitations. Agitation was much less critical when preemulsified feeds were used.