Microcrystals of AgI were deposited on alginic acid films by immersion of glass-supported silver alginate films in HI solutions. The shape or configuration of the crystals deposited on the polymer film depended on the concentration of HI: Near 0.01 M, fine whiskers of AgI, about 10μm long, covered over the surface of the polymer film in a few minutes. Near 1 M, on the contrary, a transparent thin film of AgI deposited immediately on the polymer film. The reaction rates for low HI concentrations were determined and the mechanism of the reaction discussed in terms of an ion exchange process.