Relation botween the ionic conductivity of silver halide eumision grains and the sensitivity of sulfur-sensitized emulsions at high-intensity exposure was studied by using mono-dispersed fine grain emulsions (0.2μm). The emulsions containing grains of high ionic conductivity (σ-10^-5 Ω^-1 cm^-1) exhibited no high-intensity reciprocity-law failure (HIRF) at the primitive stage, but they showed strong HIRF due to the dispersion of latent image when they were sulfur sensitized. The addition of CdC1₂ to emulsions decreased the ionic conductivity and improved this HIRF. The emulsions containing the grains of low ionic conductivity (σ-10^-8 Ω^-1 cm^-1) showed HIRF due to the limited rate of the ionic step both in primitive and sulfur-sensitized emulsions. The electron residence time in traps, which depends on the trap depth, and the time for the ionic step in latent image formation, which depends on the ionic conductivity, will influence the nucleation and growth processes of silver specks formation. If the time for the ionic step is shorter than the electron residence time, the dispersion of latent image will occur as the result of the increased nucle-ation probability. It is thought that the above situation is achieved when the emulsion of high ionic conductivity is sulfur sensitized.