The reaction of iodine with silver at temperatures from 25 to 150 °C has been studied with a quartz crystal microbalance and by x‐ray diffraction. The reaction approximately obeys the parabolic law and the reaction rate decreases with increasing temperature up to 146 °C, the transformation temperature of AgI. Crystals of &bgr;‐AgI grow with various orientations on the silver surface at 25 °C, but gradually grow with the basal planes parallel to the film surface with increasing temperature. It is concluded that the diffusion of iodine atoms through the iodide films is rate determining, and that the adsorption of iodine gas and the oriented growth of the AgI film influence the diffusion and lead to the observed temperature dependence of the reaction.