The surface acidity of raw and acid activated palygorskite clay was studied by acid–base potentiometric titration. The Gran plot method was applied for the hydroxide titration, and the total surface sites (Hs) and the average number of protons reacted per surface site (Z) of the palygorskite samples at a given ionic strength were calculated. Acid treatment increases the clay acidity and modifies its surface charge. The point of zero charge value, which was determined by the common crossing point of Z vs. the pH curves performed at different ionic strengths, decreased from 8.8 to 3.5 with the acid treatment period. For illustrating the acidic characteristics of the treated and untreated palygorskite surface, three surface protonation models were tested: (a) the one site one pKa model, ≡SOH ≡SO– H ; (b) the two sites two pKas model, ≡SIOH ≡SIO– H and ≡SIIOH ≡SIIO– H ; and (c) the one site two pKa model, ≡SOH≡SO– H and ≡SOH H ≡SO H 2 . The three surface protonation models sufficiently describe the surface properties and their evolution with the acid treatment.