In a double-jet apparatus controlled by a microcomputer, a series of formate doping emulsions of cubic AgBr grains in which formate ions were doped at different sites were prepared when a certain volume of sodium formate solution was injected into the emulsifying solution at different time intervals after the beginning of precipitation at a constant PAg. The sensitometric results from these formate doped emulsions showed: (1) that there were great differences in sensitivity between the emulsions that the same quantity of formate ions were doped at different sites of grains; (2) that relative sensitivity of doping emulsions to the corresponding undoping ones monotonically increased as the formate ions were moved closer to surface of grains; (3) that a cooperative sensitization by doping formate ions, S+Au and dye could be carried out to make a higher level of sensitivity possible without a significant increase in fog density; (4) that the formate ions doped inside AgBr grains were able to trap holes produced in dye aggregates adsorbed on the surface of grains, moreover, the closer to surface the doping ions were, the greater a sensitivity gain was.