The nitration of some tertiary phosphine oxides ArP(O)R2 in aqueous sulfuric acid has been investigated. All compounds studied react as conjugate acids. When the phosphinyl group is bonded directly to the benzene ring, high deactivation and meta-orientation is found, accompanied in most cases by some substitution at the ortho position. The substituent effects of the "quasiphosphonium" group P(OH)R2+are compared with those of structurally related systems and are discussed in terms of pπ–dπ, interactions of the oxygen and the phosphorus atom.