A scanning tunneling microscope (STM) was used to study the surfaces of galena (PbS) in aqueous solution. The influences of pH and type of purging gas on the dissolution and oxidation processes at galena surfaces were investigated. STM topographical imaging of the galena surfaces showed the development of sub-nanometer pits with increasing reaction time. Complementary atomic force microscopy (AFM) imaging confirmed the behavior. The depths (z-dimensions) of these pits correspond directly to the unit cell dimensions of galena (0.3 or 0.6 nm) and suggest that the main surface process occurring is congruent dissolution; this has been confirmed by X-ray photoelectron spectroscopy studies. The x,y-dimensions of these pits and the rate of their formation depended strongly on the pH and the type of purging gas used. Introduction of Pb2+ ions resulted in the formation of lead hydroxide colloids at the galena surface; distinct directionality is evident. Mechanisms for the initial stages of the surface chemical reactions of galena in aqueous solution are proposed and discussed in the context of flotation separation.