We observed sulfur sensitization centers, which were formed on octahedral silver bromide grains in emulsions and decorated with silver clusters by means of an amplification treatment with a special physical developer. The concentration of the centers increased with increasing the amount of the sensitizer, and reached a plateau with the amount that gave the maximum sensitivity. The concentration of the centers, which gave the maximum sensitivity, was as many as 3200 per μm², and the same as the concentration of fog centers formed with excessive amount of the sensitizer. The comparison of the concentration of the observed centers with the amount of silver sulfide formed by sulfur sensitization revealed that each sulfur sensitization center contained two sulfide ions in the emulsion with the maximum sensitivity. This result supported the idea that the sulfur sensitization centers were dimers of silver sulfide, as proposed by Keevert and Gokhale,¹ Kanzaki and Tadakuma,² and Tani.³