UV-visible spectroscopy and H NMR spectroscopy were used to investigate the kinetics and the mechanisms of the chemical imidization for low molecular weight model monoamic acid and bisamic acid. Acetic anhydride and pyridine were used as the dehydrating mixture. The reaction was found to proceed by simultaneous formation of imide and isoimide through a mixed anhydride intermediate. The isomerization of isoimide to imide was found to take place only after the amic acid starting material has been completely consumed. The isomerization was very sensitive to the solvent＇s (NMP) exposure to humidity. In humidity-exposed NMP, the isomerization was very slow, while it occurred at a much faster rate in dry MMP. The main reason for the humidity effect on isomerization is due to the hydrogen bonding of the acetate ion (a final byproduct) by water, making the acetate ion a weak catalyst. A high concentration of the mixed anhydride was detected by H NMR during in-situ monitoring of reaction in 50/50 deuterated DMSO/deuterated toluene mixture. This result indicates that the rate-limiting step of the reaction to be the deprotonation of the mixed anhydride rather than the formation of the mixed anhydride. This result also provides the explanation for the kinetic deviation observed in the UV-visible experiment when a high concentration of amic acid was used as the starting material.