The wavelength sensitivity of photodegradation of solvent-cast chitosan films exposed to monochromatic UV-visible radiation is reported. Measurements were made of changes in absorption spectra, both in the UV-visible region and in the infrared region, as well as changes in dilute solution viscosity of samples, on irradiation at selected wavelengths. Action spectra are reported for these processes. A mechanism of photodegradation based on changes in Fourier transform infrared (FTIR) spectra of irradiated chitosan is presented.