Electrokinetic properties of expanded and unexpanded perlite samples have been investigated using the microelectrophoresis technique. Perlite samples yield practically no isoelectric point in the pH range of 3 to II. Both of the perlite samples remain negatively charged in the pH range studied. The expanded perlite has a more negatively charged surface than unexpanded perlite. NaCl, KNO3, NaNO3, Na2CO3, and Na2SO4 are indifferent electrolytes for perlite whereas AlCl3 and CaCl2 change the Interface charge from negative to positive. Acid-activation has no significant effect on the zeta potential of perlite.