Reactions of triethyl or trimethyl phosphite with 2-aryl-1-phenyl-a-nitroethenes at elevated temperatures gave small yields only of non-phosphorus-containing heterocyclic compounds. Observation of the reactions by n.m.r. spectroscopy, however, showed that 3,4-diaryl-4,5-dihydro-1,2,5-oxazaphosph(v)ole-2-oxides were formed, and subsequently decomposed, under the reaction conditions. The use of t-butanol as solvent for the reaction enabled a range of novel 4, 5-dihydro-1, 2, 5oxazaphosph(v)ole-2-oxides to be prepared and isolated at room temperature. The reaction was extended by the use of dimethyl phenylphosphonite and 2-phenyl-1, 3, 2dioxaphospholan, instead of trialkyl phosphites. Further investigations, of the thermolysis of the 1, 2, 5-oxazaphosph(v)ole-2oxides and their reactions with tervalent phosphorus reagents, suggested that the compounds were not intermediates in the formation of the non-phosphorus-containing heterocycles isolated previously. The results suggested that the reaction of a tervalent phosphorus reagent with a 2-aryl-l-phenyl-l-nitroethene involves two competing pathways:- Michael-type addition of the phosphorus reagent to the nitroethene, with the possibility of subsequent ring-closure to give a 4,5-dihydro-1,2,5-oxazaphosph(v)ole-2-oxide, or deoxygenation of the nitro-group to give a vinyl nitrene, and hence nitrene-derived products. The thermolyses of two aryl 2-azidophenyl ethers in triethyl phosphate were found to give significantly different yields of products than the corresponding reactions in decalin. The possible formation of a zwitterionic nitrene-phosphate adduct as a reactive intermediate seems to be insufficient explanation of the observed results. Reaction of N-t-butylphenylnitroxyl and diphenylnitroxyl with triethyl phosphite, in ethanol or methanol saturated with lithium chloride, gave ring-chlorinated anilines. This was taken as evidence in support of a reaction mechanism in which pairs of aminyl radicals undergo an electron-transfer reaction to give an aailino-anion and a delocalised nitrenium ion, which readily undergoes nucleophilic aromatic substitution by the solvent or chloride ion. The decomposition of 1, 4-di-t-butyl-1, 4-bisphenyltetraz-2-ene in ethanol or methanol also gave ring-substituted N-t-butylanilines.