Plastic hollow waveguides (used as fibers) for infrared (IR) transmission were made from plastic tubes covered, on the internal wall, with a metal layer (Ag) and growing a dielectric thin (AgI) overlayer by direct iodination on it. The existence of several absorption lines at given wavelengths in the middle infrared (mid‐IR) region is predicted theoretically and measured experimentally. From the wavelengths of absorption lines the thickness of the AgI film has been computed. The average thickness of the AgI in the hollow waveguide increased with the iodination time and with the concentration of the iodine solution. The crystal size of the AgI was increased with the increase of the AgI thickness. By controlling the iodination process it was possible to make waveguides which can be employed as filters for various wavelengths of the transmitted mid‐IR radiation.