Comparison of the increase in sensitivity of AgBr emulsion layers caused by their storage in Ar gas at 70°C for 72 h with that caused by reduction-sensitization of the same emulsions revealed that R centers of reduction-sensitization were formed on the emulsion grains during the storage. The driving force for the electron transfer from gelatin to AgBr grains on storage for formation of R centers was confirmed by ultraviolet photoelectron spectroscopy (UPS) measurement, whereby the Fermi level of gelatin was higher than that of AgBr. It was however proposed for the formation of R centers on storage that the electron transfer took place by a two-electron process without creating any free electron or any single silver atom in the AgBr grain. Fog centers as well as reduction-sensitization centers were formed on sulfur-plus-gold-sensitized grains during storage. A stabilizer 4-hydroxy-6-methyl-1,3,3a-7-tetraazaindene (TAI) depressed the sensitivity increase and fog formation in layers of sulfur-plus-gold-sensitized AgBr emulsions on storage. The observation provided evidence for the mechanism of stabilization of emulsion layers by AI, according to which TAI depresses sensitometric change on storage by preventing formation of reduction-sensitization centers.