The reactions of tervalent phosphorus reagents with a number of aromatic nitro-compounds have been investigated. While it was expected that such reactions would give further examples of the deoxygenation and further reaction of the nitro-group, possibly via a reactive nitrene intermediates in fact, two, different, novel displacements of the nitro-group were found. In the first case, reaction of o-dinitrobenzene and related compounds with tervalent phosphorus reagents gave a series of phosphonates, phosphinates and phosphine oxides, in high yield, with the phosphorus atom bonded directly to the aromatic system, generally with an o-nitro-group still present in the final product. A mechanism involving nucleophilic aromatic substitution by the phosphorus reagent, with the displacement of the nitro-group as ethyl nitrite, was suggested. In the second case, reaction of p-nitrotoluene and p-ethylnitrobenzene, for example, with triethyl phosphite, gave low yields of diethyl p-tolyl- and p-ethylphenylphosphonate. As there is no activation of this nitro-group towards aromatic nucleophilic substitution, a mechanism involving attack on an oxygen of the nitro-group by the phosphorus reagent, with sub- sequent rearrangement through a four-membered intermediate was proposed. The other products of the reactions of o-alkyl-nitrosobenzenes and alkylnitrobenzenes with trialkyl phosphites were, in general, indicative of the participation, in the reaction, of a nitrene intermediate. The similarity of the products formed to those found in established examples of the reactions of nitrene intermediates, was noticed. The mechanism of this reaction was discussed in detail.