In this study, the influence of pH, electrolyte concentration, and type of ionic species on the electrokinetic properties (zeta potential and electrokinetic charge density) of the acid-activated montmorillonite mineral have been investigated using the microelectrophoresis method. The electrokinetic properties of acid-activated montmorillonite dispersions have been determined in aqueous solutions of mono-, di-, and trivalent salts and divalent heavy metal salts. Zeta potential experiments have been performed to determine the point of zero charge (pzc) and potential determining ions (pdi). The zeta potential values of the acid-activated montmorillonite particles were negative and did not vary significantly within the pH range studied. Acid-activated montmorillonite dispersions do not have point of zero charge (pzc). The valence of the electrolytes has a great influence on the electrokinetic behavior of the suspension. A gradual decrease in the zeta potential (from -25mV to -5mV) occurs with the monovalent electrolytes when concentration increased. Divalent and heavy metal electrolytes have less negative z-potentials due to the higher valence of ions. A sign reversal of z-potential has been observed at AlCl3, FeCl3, and CrCl3 electrolytes (potential determining ions) and zeta potential values have had a positive sign at high electrolyte concentrations. The electrokinetic charge density of acid-activated montmorillonite has shown similar trends for variation in mono- and divalent electrolyte solutions. Up to concentrations of ca. 10-3M, it has remained practically constant at approximately 0.5x10-3 C m-2 For higher concentrations of monovalent electrolytes more negative values (-16x10-3 C m-2) were observed. It has less negative values in divalent electrolyte concentrations according to monovalent electrolytes (-5x10-3 C m-2). For low concentrations of trivalent electrolytes, the electrokinetic charge density of montmorillonite particles is constant, but at certain concentrations it rapidly increased and changed its sign to positive.