Electrophoretic light scattering (ELS) is used to determine the zeta potential of O/W emulsions of five commercial cutting oils. AlCl3, MgCl2, CaCl2and NaCl were added as electrolytes to the emulsions at concentrations varying from 10−4mol/dm3to 10−1mol/dm3in order to study the influence of electrolyte concentration and cation charge on the zeta potential of the emulsions and to explain their stability.