The amphoteric properties of hydrous ZnS surfaces as well as an ion exchange reaction between surface coordinated Zn2+ ions and H+ ions of the aqueous phase have been established. The measurements were performed as potentiometric titrations in 0.1 M NaClO4 medium at 25°C. Besides measuring -log[H+] by using a glass electrode, -log[S2-] was recorded (Ag/Ag2S selective electrode). In addition, the total concentrations of H+, Zn2+, and S were analyzed as a function of -log[H+] of the bulk solution. Synthetically prepared ZnS and the mineral sphalerite were studied. Bro¨nsted acidity constants pK1s = 6.91 and pK2s = 10.28 for synthetic ZnS, and pK1s = 7.14 and pK2s = 10.29 for sphalerite, were established, which implies -log[H+] of zero proton condition equal to 8.60 and 8.72, respectively. Furthermore, the ion exchange reaction ≡SZn + 2H+ ↔ ≡SH2 + Zn2+ was characterized within the range 4 ≤ -log[H+] ≤ 7, with log K = 9.59 and 9.65. Experimental data were evaluated on the basis of the constant capacitance model by using the computer program FITEQL. The "best" fit to experimental data was obtained by assuming no charge dependence of K1s and K2s.