The attenuated total reflection/FTIR spectra of the (100) galena/electrolyte interface were measured in situ for the first time in nitrogen-saturated borate buffers (PH 9.2) of two concentrations in the potential region of -0.5 to +0.7 V (SHE), simultaneously with the voltammograms. The experiments were conducted on the electrode previously reduced at -0.5 V and the nonreduced electrode as well. Not only vibrational bands but also monotonic absorption of free carriers was analyzed. It was concluded that at the first step galena is oxidized by the reaction PbS + 2xh(+) --> Pb1-xS + xPb(2+), which is followed by the reaction PbS + 2h(+) double right arrow Pb2+ + S degrees . Pb(OH)(2) is formed by the precipitation mechanism. The Pb(OH)(2) yield of galena oxidation increases as either the buffer concentration or the time of the galena reduction is increased, which was explained by an increase in the potential drop in the Helmholtz layer. When a more positive bias potential is applied, lead sulfite and thiosulfate are formed by the reactions PbS + 3H(2)O double right arrow PbSO3 + 6H(+) + 6e and PbS + S degrees + 3H(2)O --> PbS2O3 + 6H(+) + 8e, respectively. These processes were correlated with the galena floatability.