It has been shown in the previous works that the addition of the very small amounts of hexachloroiridium complex at the precipitation stage of silver halide eliminates the high intensity reciprocity failure of the emulsion and increases its sensitivity at high intensity exposure. On X-ray exposure, a comparatively large amount of energy can be transfered to a single grain by the passage of a single electron. In this respect, X-ray exposures resemble the exposures of very high intensity light. Therefore it is expected that the sensitivity of the emulsuons to X-ray might be influenced by the addition of the iridium complex. In this paper the effects of the iridium complex on the X-ray sensitivity of the photographic emulsions are studied and compaired with the effects of lead and thallous salts which have been known as a sensitizer for X-ray emulsions. The analogous behaviors are observed on three kinds of the metal salts.