The influence of different factors on the overall mass transfer coefficient of liquid-liquid extraction was investigated, employing single-drop experiments with mass transfer in either direction. The chemical system of toluene-acetone-water was used with the advantage of high accuracy and repeatability when using gas chromatography for analysis. Three different types of surfactants were used as contaminant agents, and their effectiveness in retarding the mass transfer appeared in the order cationic < anionic < nonionic. Addition of contaminants did not change the trend of variation with drop size for the concentration range investigated in this study; however, a number of small drops demonstrated rigid behavior at high contaminant concentrations. The rate of mass transfer is greater in the dispersed to continuous phase direction. An empirical correlation was developed here in conjunction with a combined model which describes the variations of the mass transfer coefficient. The variation of physical properties was also taken into account.