Gelatin films containing cobalt chloride were investigated to obtain information on the humidity dependence of optical absorbency in view of the coordinate state of cobalt ion and the polymer structure. The visible absorption spectra of the films exhibited that, with increasing relative humidity (RH), the absorption bands between about 550 and 750 nm decreased, accompained by sharpening of the 693 nm peak. Further, the decrease of cobalt chloride in gelatin resulted in decreased absorption intensity of the 693 and 668 nm peaks, more than the overall lowering in intensity. The infrared spectral data indicated that most of the carboxyl groups of gelatin are linked to the complex even in the strongly hydrated state. In addition, the water content of the films containing cobalt chloride is a little less below 42% RH, but is much greater above 62% RH than that of the original gelatin film. These results indicated that there are cross-linked networks, composed of the hydrogen bonding and the coordination bond via the complex in the dry film; as water molecules are adsorbed by the film, they gradually destroy these bonds and swell the polymer. © 1993 John Wiley & Sons, Inc.