Hydrogen sulfide (H₂S) and carbonyl sulfide (OCS) are the principal atmospheric corrodents for silver and copper. To investigate the mechanisms by which the sulfidation reactions occur, the two metals have been exposed to each of the sulfurous gases under a wide range of relative humidities. The sulfidized ion scattering samples were analyzed by scanning electron microscopy, X-ray fluorescence spectroscopy, low energy ion scattering spectrometry, spark source mass spectroscopy, Auger spectroscopy and X-ray photoemission spectroscopy. The results demonstrate that sulfidation of both metals by both gases is strongly dependent on the relative humidity, that negligible carbon or oxygen is incorporated into the sulfide layer during OCS sulfidation and that the products of sulfidation by either H₂S or OCS are Ag₂S and Cu₂S. These results are used to develop a general mechanism for atmospheric sulfidation in which the adsorption of the gas into the surface water layer is followed directly by dissociative coordination with metal atoms. Experiments capable of confirming some of the steps in this mechanism are proposed.