The kinetics of the styrene emulsion polymerization using n-dodecyl mercaptan as chain-transfer agent was studied. It was found that the chain-transfer agent (CTA) had no effect on polymerization rate but substantially affected the molecular weight distribution (MWD). The efficiency of the CTA in reducing the MWD was lowered by the mass-transfer limitations. The process variables affecting CTA mass transfer were investigated. A mathematical model for the process was developed. The outputs of the model include monomer conversion, particle diameter, number of polymer particles, and number-average and weight-average molecular weights. The model was validated by fitting the experimental data.