The photodeterioration of silk and cotton has been studied by crystallinitic procedures, e.g., infrared spectroscopy, x-ray diffraction, or measurements of tensile strength, elongation before breaking, colors (CIELAB), copper numbers, and amino acid compositions. The crystallinities of silk and cotton did not change by the irradiation of light from mercury or xenon lamps and natural sunlight. The tensile strengths and the elongation values before breaking, in the case of silk samples, showed a considerable decrease when compared to those for cotton samples at the same irradiation doses. The contents of carbonyl groups in cotton samples after irradiation were changed very little. The amounts of amino acids in silk, on the other hand, decreased considerably, especially tyrosine and tryptophan.