Various methods of alkylation of the >PO2−,>-PS2− and >POS− ions have been investigated. Ethyl miethylphosphonate reacted with 2-methyloxiran to give ethyl 2-hydroxypropyl methylphosphonate and with p-nitrobenaonitrile oxide to give ethyl 0a-hydroxyimino-4-nitrobenzyl methylphosphonate, a new type of organophosphorus ester, which was shown to hydrolyse in acid solution about 107 times faster than a simple phosphonate ester. A kinetic study of this remarkably fast hydrolysis suggests a mechanism involving neighbouring group participation of the hydroximino- moiety. Alkylation of ethyl methylphosphonate was not achieved by reaction with oxasiridines. Reaction of dihgxylphosphinic acid with p-nitrobenzonitrile oxide gave a-hydroxyimino-4-nitrobenzyl dihexylphosphinate. This ester was stable in acid solution but both the a-hydroxyimino-4-nitrobenzyl dihexylphosphinate and phosphinate esters decomposed at pH5 to give p-nitroaniline. A possible route of decomposition involving a Lossen rearrangement is suggested. Alkylation products were not isolated when OO-diethyl phosphoro-thioate and -dithioate reacted with p-nitrobenzo-nitrile oxide; the products obtained indicate that an alkylation, similar to that of the >PO2− ion, had occurred but subsequent facile decomposition of the neutral anhydrous esters had taken place. A possible application of the realkylation and subsequent decomposition of the >PO2− ion, brought about by reaction with nitrile oxide, in the reactivation of "aged" phosphonylated cholinesterase is discussed. The reaction of propyl iodide with dialkyl phosphoramidates has been shown to cause an alkyl exchange process on both oxygen and nitrogen atoms, together with P-N bond fission. The phosphorylation of phenol to give triphonyl phosphate was accomplished by heating with diphenyl N-propylphosphoramidate in the presence of propyl iodide.