The fine structures of polynosic fiber, high-wet-modulus fiber, high-tenacity rayon and normal rayon were examined by various methods and the following results are obtained:(1) X-ray and acid hydrolysis studies indicete that polynosic. fiber aud high-wet-modulus fiber have higher crystallinity and show lower rates of recrystallisation during acid hydrolysis than in high-tenacty rayon and normal rayon. Further-more the infrared spectroscopic results of these rayon fibers show that there exists a linear relationship between the crystallinity and the apparent molar extinction coefficient α of the absorption band at 1375 cm-1, and α s of polynosic fiber and high-wet-modulus fiber are higher than in those of high-tenacity rayon and normal rayon. Next, increasing rates of α of polynosic fiber and high-wet-modulus. fiber during acid hydrolysis are lower than those of high-tenacity rayon and normal rayon. These infrared spectroscopic results show a good agreement with the conclusions which are derived from acid hydrolysis and X-ray methods.(2) The degrees of swelling by NaOH solution of polynosic fiber and high-wet-modulus fiber are lower, compared with those of high-tenacity rayon and normal rayon. Infrared spectroscopic studies indicate that the increasied rate of α of the absorption band at 1375 cm-1 during the alkali treatment of polynosic fiber is lower, compared with those of other fibers. This result suggests that the rate of decrystallisation of polynosic fiber during the alkali treatment is lower than those of other fibers.