Dependence of fog on chemical sensitization and iodide amount in cubic AgBr emulsions has been studied. Similar fog curve shapes were obtained from the iodide-covered AgBr emulsions by varying the iodide amount and the sulfur-sensitized AgBr emulsions by varying the sulfur amount. Subsequent gold sensitization of the two kinds of emulsions produces two fog peaks in the curves with varying the iodide or sulfur amount. The surface properties of the emulsion grains have been discussed with reference to fog formation. The origin of fog is attributed to the reactivity difference between the chemical site and the bulk crystal surface. The reactivity sequences of different chemical sites were derived: Ag₂S > AgI > AgBr > AgCl and Au_n > Ag_n, etc. Direct electron transfer development (DETD) and indirect electron transfer development (IETD) were supposed to be the two processes in development. Based on the proposed mechanisms, the main experimental results were successfully explained with a distribution model of chemical impurities at the cubic AgBr grain surface.