In this experimental study, the presence effect of Aniline surfactant on mean drop size, D-32, was investigated in a horizontal mixer-settler. For this purpose, two 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 the impeller speed and the holdup on the drop size distribution were examined without Aniline. Afterwards the same investigation was performed in the presence of Aniline. For both experimental series, a new modified correlation for D-32 was presented. The results revealed that D-32, in the absence of Aniline, depended on the impeller speed with a power low function, having an exponent of -1.208 and being in agreement with the Hinze-Kolmogorov's theory. In the presence of Aniline, a greater reduction in D-32 was observed when the impeller speed increased. Moreover, the increasing rate of D-32 diminished with the increase in the holdup. (C) 2009 Published by Elsevier B.V.