AbstractA single stage mixer—settler was used to investigate the effect of surfactant on the mass transfer rate in the system water—HNO3—30 vol.% TBP/dodecane. The interfacial tension of this system first falls then rises with increasing sodium lauryl sulphate (SLS) concentration. The addition of SLS makes the stage efficiency, which is closely related tokha, the product of the individual mass transfer coefficient of HNO3in the aqueous phase, and average interfacial area per unit volume of mixing chamber, to increase significantly due to an increase in the value ofa.A maximumkhavalue of 0.53 litres−1, a minimum value of interfacial tension, and phase inversion which converted the aqueous phase from continuous to dispersed were observed at around the critical micellar concentration (100 parts 10−6) of SLS in the system of an aqueous to organic phase rat