High temperature deoxygenation of 2-aryl-l-phenylnitroethenea with triethyl phosphite gave products considered to arise from the vinyl nitrene. Comparison of the products with those obtained by deoxygenation of the corresponding 3,4-diarylt 5-dihydro-2-oxo-1, 2, 5-oxazaphosph(v)olcs showed that these phosph(v)oles were likely intermediates in the nitro deoxygenation reaction. In two cases, the corresponding 1,2,5-oxazaphosph(v)oles lacking the N-oxide function were isolated as stable solids. This suggested the possibility of an alternative deoxygenation mechanism operative in the case of nitroethenes. Thermolysis of the 1,2,5-oxazaphosph(v)oles provided good evidence for the intermediacy of the vinyl nitrene. Convincing evidence for the involvement of the nitrosoethene could not be obtained, and all attempts to synthesise 1,2-diphenyl-l-nitrosoethene met with failure. A mechanism involving the two oxazaphosoh(v)oles was shown to be feasible, however, although it did not appear to be the only mechanism operating. The spectroscopic features of the oxazaphosph(v)oles were discussed in detail, in particular the variable temperature NMR spectra of the 2-oxo-l,2,5-oxazaphosph(v)oles. These were discussed in terms of the pseudorotation processes taking place, and free energies of activation were calculated for the oxazaphosph(v)oles derived from dimethyl phenylphosphonite. The hindered rotation of a 1,2,5-oxazaphosph(v)olo was also studied. The reactions of two 2-alkyl-1,2-diphenylnitroethenes with triethyl phosphite were investigated, and steric effects on the formation of 2-oxo-1,2,5-oxazaphosph(v)oles discussed. It was found that the high temperature reaction led to an unusual phosphate-induced rearrangement without deoxygenation of the nitro group. The acid catalysed hydrolyses of the 2-oxo-1,2,5-oxazaphosph(v)oles were studied. The results were interpreted in terms of competitive protonation of the N-oxide function and the apical methoxyl group. The principal products were ketone phosphonates arising via a lief reaction. In one case where the phenyl ring contained a p-methoxyl group, a completely different reaction occurred resulting in the formation of a spirodienone.