Rolled Cu or Ag sheet was suspended in a hygrostated reaction vessel at 5, 23, or 38 degress and H₂S at 0.02 atm., 0 at 0.10 atm., and N at 0.38 atm. were admitted. The amount of combined S was determined gravimetrically or electrometrically and plotted versus time. The morphology of the sulfides was studied in an electron microscope by means of C replicas shadowed with Pt. The sharp increase in reaction rate with relative humidity in the ranges of 0-30 and 75-100% is explained in terms of capillary condensation of H₂O in the surface crevices. The activation energy of the reactions in a dry atmosphere is 60 kj. henius equation. -- AATA