The rates of formation and decomposition of a series of phosphylated oximes derived from P2S (2-hydroxy-iminomethyl-1-methylpyridinium methane-sulphonate) have been measured. The rates of inhibition of AChE by these phosphylated oximes and the parent (and related) organophosphates have also been measured. Possession of these rate data now permits a detailed analysis of the reactivation of phosphylated AChE by P2S to be made (see following paper).