Reduction sensitization with various sensitizers, silver digestion, and hydrogen hypersensitization were applied to octahedral and cubic silver bromide emulsion grains to form P centers, identified by the observation of sensitivity increase and photoconductivity decrease caused by the centers, and absorption band characteristic of the centers. It was found that P centers decreased the ionic conductivity of the grains, which was proportional to the concentration of interstitial silver ions. By taking into account the fact that silver ions at kink sites are in equilibrium with interstitial silver ions, we propose that a P center was formed adjacent to a silver ion at a kink site and prevented it from going into an interstitial position in an emulsion grain. It is further proposed that a P center of a silver dimer is not merely Ag₂ in adjacent to Ag⁺ at a kink site, but a center as expressed by Ag₃⁺. The new model for an R center and a P center was successfully applied to the explanation of the formation, property, and behavior of silver clusters of photographic interest.