The formation of silver clusters due to reduction of silver ions by gelatin during precipitation process of AgBr grains in emulsions was studied from the point of view that it should take place when the potential of the silver half-cell (E_Ag) was more positive than the potential of the gelatin half-cell (E_Gel) in a reaction solution. Although the sensitivities of the prepared emulsions were independent of E_Ag of the bulk reaction solutions, they increased with increasing pH values, and reached the sensitivities of the corresponding reduction sensitized emulsions and hydrogen hypersensitized ones. These results indicate that the formed clusters were reduction sensitization centers. It is proposed that silver cluster formation takes place, not in the bulk reaction medium, but in the region into which an aqueous solution of silver nitrate has been introduced for precipitation, since this is the only region where E_Ag is more positive than E_Gel in a reaction vessel.