Part 1: The occurrence of bimolecular nucleophilic aromatic substitution has been established in the reaction of o-dinitrobenzene with triethyl phosphite and diethyl methylphosphonite in acetonitrile solution. There is a fine balance between the customary deoxygenation and nucleophilic substitution of aromatic nitrocompounds. The role of the o-nitro group in facilitating the nucleophilic substitution has been discussed in terms of "built-in solvation" of the reaction centres. The exclusive formation of ethyl nitrite in the reactions has been shown to be compatible with the dealkylation of a quasi-phosphonium intermediate by nitrite ions. Part 2: A series of alkyl a-hydroxyimino-p-nitrobenzyl alkylphophyl adducts have been synthesised from phosphorus-containing acids and p-nitrobenzonitrile oxide. Their solvolytic behaviour has been examined. Hydrolysis occurred in acid solution at 25°with anchimeric assistance of about 2x107 by the neighbouring oximino group, to yield the alcohol of the ester moiety. Exclusive phosphorus-oxygen fission occurred. P-Nitroaniline was formed in alkaline solution by intramolecular attack by the oximate anion, followed by a Lossen rearrangement. Evidence is presented for the intermediary of pentacovalent intermediates, and the relative reactivities of the adducts are explained by the constraints necessary for pseudo-rotation to allow further reaction to form the products in alkaline solution. The acid hydrolysis of ethyl pinacolyl methylphosphonate was shown to proceed via a unimolecular mechanism with the formation of a carbonium ion in 50% aqueous dioxin at 100 °.