The stability of three oximes, HI-6 [(4-carbamoyl-2`[(hydroxyimino)-methyl]- 1,1`-oxydimethylenedi-(pyridinium chloride)], HI-CN [(4 cyano-2`-[(hydroxyimino)-methyl]-1,1`-oxydimethylene-di-(pyridinium chloride)], and 2-PAM [pralidoxime chloride] in aqueous solutions was evaluated by HPLC assay. The rate of degradation is dependent on the pH as well as the temperature at which the solution is stored. The optimum pH for the stability of these oximes was pH 2 to 3. The degradation rate constant for 2-PAM (k at 70℃, 2.07×10^(-4)/hr; E_a value, 27.2 ㎉/㏖) was smaller than those for bis-pyridinium oximes, HI-6 (k at 70℃, 3.38×10^(-3)/hr) and HI-CN (k at 70℃, 8.66×10^(-3)/hr; Ea value, 20.7 ㎉/㏖). In mechanistic analyses, it was found that HI-CN was decomposed through not only the hydrolysis of nitrile group but also the cleavage of methylene ether bridge, in contrast to HI-6 which was degraded mainly through the cleavage of methylene ether bridge.