Heavy hydrogen hypersensitization formed P centers, which were silver nuclei acting as electron traps on octahedral silver bromide grains in coated emulsion layers. The number of P centers was evaluated by measuring the diffuse reflectance spectrum of a stack of 10 coated emulsion layers. Sulfur sensitization centers increased the rate of formation of P centers, and decreased its activation energy. The observed activation energies were 0.87 eV in a primitive emulsion and 0.52 eV in an excessively sulfur sensitized one. This result supports an idea that sulfur sensitization centers formed on silver bromide emulsion grains catalyze silver nuclei formation, and that this is one of the essential functions of sulfur sensitization centers.