The striking distinctions of oximes. such as 2-PAM,LuH6,TMB₄ and HI-6. in reactivating sarin-,soman-,tabun- and VX- phosphorylated acetylcholinesterase (AChE) imply that there are factors which hinder phosphorylated enzymes from being reactivated by some oximes before aging. To investigate this phoblem,a comparison of in vitro reactivation of these phosphorylated enzymes by H series oximes,HI-6,HGG-42 and their 4-oxime isomers.was made. Results showed that,in tabun experiments,although there was no reactivation by the two 2-oximes (HI-6 and HGG-42)，28%-45% reactivations were observed by their 4-oxime isomers. In soman case the results were opposite. HI-6 and HGG-42 showed remarkable reativation. but their 4-oxime isomers were ineffective. For sarin- and VX- phosphorylated AChE, all oximes showed high effctiveness. These results indicated that there are conformational differences among the active centre of the above phosphorylated enzymes. The influences of AChE allosteric agents showed that C₁₀ significantly increased the effects of TMB₄ in sarin, soman and VX experiments, however. it reduced the effect of TMB₄ significantly in the tabun case. Propidium has no influence on the reactivation of sarin, soman- and VX-phosphorylated AChE, but it significantly reduced the effect of TMB₄ on reactivating tabun-phosphorylated enzyme. These results confirmed that the conformation in the active centre of tabun-phosphorylated enzyme is obviously different from that of sarin-，soman- and VX- phosphorylated enzymes.