In this study, the effect of Aniline and SDS (Sodium Dodecyl Sulfate) surfactant on mean drop size, D-32, was investigated in a horizontal mixer-settler. For this purpose, three series of experiments were conducted in a single stage mixer-settler on the liquid-liquid dispersion of a toluene-water system. At first, the effects of impeller speed and hold-up on the mean drop size were examined without any surfactant. Afterwards the same investigation was performed in the presence of SDS (Seri 1) and then in the presence of Aniline (Seri 2). The results revealed that D-32 in Aniline system is larger than D-32 in SDS system. In addition, the results show that D-32 in the presence of Aniline depends on the impeller speed with a power low function, having an exponent of -1.11 which has a good agreement with Hinze-Kolmogorov's theory. (C) 2013 Published by Elsevier B.V. on behalf of The Institution of Chemical Engineers.