A splittable surfactant, Triton SP-190, was used to evaluate the effects of acid treatment on the mass transfer rate of an extraction process and on the interfacial tension-lowering activity of a system containing this surfactant. Equilibrium and dynamic interfacial tensions at the interface Of CCl4 and the aqueous phase containing surfactant were measured by using pendent drop tensiometry enhanced by video digitization. A single-drop extraction apparatus was used to obtain the extraction percentage of acetic acid from the dispersed CCl4 droplets to the aqueous phase. The results indicate that the inorganic acid treatment can inhibit the dynamic and equilibrium interfacial tension-lowering activity of Triton SP-190. The mass transfer resistance induced by the addition of Triton SP-190 can also be reduced by the pre-treatment of acid. The effectiveness of acid treatment on both properties was greater at low pH values, lower surfactant concentrations, and longer treatment times. With HCI treatment, the equilibrium interfacial tension was not able to increase to the value of a surfactant-free system, but approached a maximum value which was independent of the pH value, but dependent on surfactant concentration. On the contrary, the extraction percentage, which has decreased due to the presence of surfactant, can be recovered completely to that of a surfactant-free system by acid treatment. The acid-treatment time required to achieve a significant recovery of mass transfer rate was much longer than that required to recover the interfacial tension. The present results also demonstrate that the constituents contained in an acid-treatment system had different effectiveness in affecting the interfacial tension and mass transfer rate due to the different mechanisms involved. (C) 2004 Society of Chemical Industry.