Physical properties have been compared among AgCl, AgBr, and AgI grains. The formation energy of interstitial silver ions through surface kink sites is in the order of AgCl, AgBr, and AgI, while the activation energy of the migration of interstitial silver ions is in the reverse order. This result is ascribed to the difference in ionic character among them, and leads to the fact that the concentration of interstitial silver ions acting as shallow electron traps is in the order of AgI, AgBr, and AgCl. This result is therefore responsible for the fact that the concentration of photoelectrons, as measured by means of a 35 GHz microwave photoconductivity apparatus, is in the order of AgCl, AgBr, and AgI. Since the solubility in water is in the order of AgCl, AgBr, and AgI, it is considered that the concentration of dissolving impurity ions acting as positive hole traps is in the order of AgCl, AgBr, and AgI, and that the silver ion concentration in the vicinity of latent image centers on the grain surface is in the same order. Although these facts make it more difficult to form latent image centers on AgI grains than on AgBr and AgCl grains in conventional photographic materials, in which the concentration of silver ions is kept low for their stability, it is indicated that AgI grains are suitable for photothermographic materials, in which AgI grains are placed in the environment with higher concentration of silver ions than in conventional ones, and are fixed during thermal development, contrary to AgBr and AgCl grains.