When the films of silver alginate prepared on glass plates were immersed in HCl solutions ranging in concentration from 10^-3 to 6 M, microcrystals of AgCl were formed on the films of alginic acid. With the aid of a scanning electron microscope, it was confirmed that the shape or configuration of the crystals varied widely with the concentration of HCl, but it was definite at a definite concentration. The primary process of this reaction could be described as an ion exchange between Ag⁺ and H⁺ in the film. It was concluded, on the basis of the kinetic studies, that the rate-determining step was a diffusion process of Cl^- through the solution to the surface of the film, and that the flux of Cl^- was a main factor governing the shape or configuration of the crystals.