We prepared chitosans with various degrees of deacetylation (DDAs) by mixing completely deacetylated chitosan and acetic anhydride at room temperature without serious degradation and O-substitution. We obtained a standard curve to measure DDA by plotting the IR absorbance ratio of A(1560)/A(2880) against the known DDAs (from 1-100%) of 10 specimens. The effect of DDA on the critical concentration (C*) of chitosan/dichloroacetic acid solutions required to form mesophase was investigated by optical methods. A maximum C* value of 23 wt % appeared at a relative medium DDA (similar to 20%). The effect was explained by the disordering of chains with medium composition ratios of the copolymer of glucosamine and N-acetyl glucosamine.