Water-soluble hydroxypropyl chitin (HPC) derivatives were prepared by the reaction of chitin with propylene oxide in homogeneous conditions using a dimethy-lacetamide/5% lithium chloride solvent system. The reaction conditions for a proper substitution of the hydroxypropyl group were an 80 degreesC reaction temperature, 24-h reaction time, and 48-h aging time. H-1-NMR and C-13-cross-polarization/magic angle spinning NMR spectroscopy were used for determining the substitution value and reaction site of substitution. It was found that the substitution reaction occurred mainly at the C-6-OH group in the chitin molecules and water-soluble chitin derivatives could be obtained at a substitution value higher than 0.35, which is known as a critical substitution value. X-ray diffraction analysis and FTIR spectroscopy showed that the structural characteristics of HPC derivatives were dependent on the substitution value. As the substitution value of the HPC derivative increased the alpha -chitin crystal structure changed to the water-soluble beta -chitin form in higher substitution values. Differences in the interchenic hydrogen bonding, interplanar spacing of the crystal lattice plane, and crystallinity can explain the structural changes upon substitution. The thermal decomposition temperature was also related to the characteristics of the crystalline structure, depending on a critical substitution value.