The surface charge behaviour of unexpanded and expanded perlite samples in KNO3 and NaCl solutions were investigated as a function of pH and ionic strength. The solutions of KNO3 and NaCl ranging from 10(-3) to 1.0 M were used. The potentiometric titration method was used to determine the surface charge of perlite samples. It was confirmed that the perlite samples had no the point of zero charge and was negatively charged in the pH range of 3-10. The double extrapolation method was used for determining the intrinsic equilibrium constants for simple ionization and complex ionization reactions. The values obtained are pK(a2)(int) = 2.5 and p*K-K+(int) = 2.3 in KNO3 solutions and pK(a2)(int) = 3.0 and p*K-Na+(int) = 2.4 in NaCl solutions for unexpanded perlite, and pK(a2)(int) = 2.6 and p*K-K+(int) = 2.4 in KNO3 solutions and pK(a2)(int) = 2.7 and pK(Na+)(int) = 2.4 in NaCl solutions for expanded perlite.