The effects of surfactant concentration, impeller speed, and holdup on mean drop diameter, D-32, during emulsification have been studied in a mixer-settler. Two well-defined regions for the dependence of D-32 on surfactant concentration were observed. At low surfactant concentrations, D-32 decreased significantly with an increase in surfactant concentration, whereas D-32 did not depend on surfactant concentration at high surfactant concentrations. The decreasing rate of change of D-32 with impeller speed and the increasing rate of change of D-32 with holdup both decreased when a surfactant was added to the system. Moreover, an empirical correlation has been derived to predict D-32.