The initial mass-transfer rate of acetic acid from the aqueous to the organic phase with tri-n-octylamine and toluene as solvents has been studied in a constant interfacial cell as a function of the stirring speed, concentrations of the transfer species, and chemical adsorption layers. For a detailed understanding of the mass-transfer process, the electrophoretic mobility of oil-in-water droplets with different interfacial charges has been measured with a ZETA sizer as a function of the pH value. It could be proven that a change of the interfacial charge influences the xi potential of the dispersed oil drops and thus the mass transfer of the acetic acid. A,kinetic model and its electrostatical extension is introduced to describe the interfacial reactions and mechanism, when using a thermodynamic-consistent activity coefficient model.