Proton adsorption at hydrous sulfide mineral surfaces has been investigated. Potentiometric titrations and solubility studies of freshly precipitated PbS, commercially available PbS and ZnS as well as the natural minerals galena and sphalerite were performed. An experimental method for accurate determination of the concentration of reacted protons at the metal sulfide surface is presented. In slightly acidic solutions an ion exchange reaction between protons and surface metal ions was discovered leading to quite high metal ion concentrations in the bulk solution.