Electron tunneling spectroscopy (ETS) makes possible the study of electronic structures of conductive mineral surfaces and can be applied under ambient or in-situ conditions rather than in a vacuum. We use ETS to study fresh, oxidized, and aqueous-Au-treated PbS (001) surfaces in order to demonstrate the strengths and limitations of the technique in the context of a geochemical application. Experimental tunneling spectra are in qualitative agreement with tunneling theory. Spectra for Au, fresh PbS, and oxidized PbS are different (primarily because of work function differences) and can be used to distinguish between, for example, Au and PbS when they are present on one surface. ETS is thus useful as a compositional probe with high spatial resolution. Spectra for fresh PbS show a small peak at about +250 mV that corresponds to conduction band states with mixed Pb and S character. Both the ETS spectra and STM images suggest that these states are lost upon oxidation. Au precipitated on PbS from aqueous solutions as isolated islands looks identical to metallic Au in ETS.