Silk fibroin (SF) fiber from the Bombyx mori silkworm was treated with a 1.23 N iodine/potassium iodide (I-2-KI) aqueous solution, and the structure and physical properties were investigated to elucidate the effects of the iodine treatment. The SF fiber absorbed polyiodide ions such as I-3(-) and I-5(-) by immersion in the I-2-KI solution, and the weight gain of the SF fiber increased with the treatment time; it became saturated at about 20 wt % after 40 h. The results of the weight gain, Fourier transform infrared spectroscopy, and X-ray diffraction measurements suggested that polyiodide ions mainly entered the amorphous region. Moreover, a new sharp reflection in the meridional direction, corresponding to a period of 7.0 angstrom, was observed and indicated the possibility of the formation of a mesophase structure of ss-conformation chains. Dynamic viscoelastic measurements showed that the molecular motion of the crystalline regions at about 220 degrees C was enhanced and shifted to lower temperature by the introduction of polyiodide ions. This indicated that the iodine component weakened the hydrogen bonding between the SF molecules forming the ss-sheet structure and caused molecular motion of the crystal to occur more easily with heating. With heating above 270 degrees C, the iodine component introduced intermolecular crosslinking to SF, and the melt flow of the sample was inhibited. (c) 2006 Wiley Periodicals, Inc.