Previous works of this series, it has been shown that the addition of a small amount of hexachloroiridate at the precipitation of silver halide eliminates the high intensity reciprocity law failure of the photographic emulsions. In this paper, the situation of iridium ion in the photographic emulsions was studied by means of some analytical techniques. The potentiometric titration of potassium hexachloroiridate with silver nitrate indicated that hexachloroiridate reacting with silver nitrate formes the precipitate of silver hexachloroiridite and it has considerable tendency to co-precipitate with silver bromide. According to the determination of iridium contained in silver bromide prepared in the presence of hexachloroiridate, majority of iridium presented is co-precipitated with silver bromide. And the lattice constants of the silver bromide crystals determined by X-ray diffraction are decreased with the increasing of co-precipitated iridium. It is considered that hexachloriridate added at precipitation of silver halide emulsions formes silver hexachloroiridite and distributes in the silver halide grain in form of solid solution. On this point of view, the mechanism of elimination of high intensity law failure by the iridium salt is discussed.