Third order hyperpolarizabilities were calculated using the Huckel Hamiltonian and third order perturbation theory for a series of phosphonitrilic compounds, (X²P-N)ⁿ, as a function of bond length alternation, ligand substitution and backbone conformation. Phosphonitrilic compounds show hyperpolarizabilities comparable to those reported for organic species, and are modulated by ligand group electronegativity. In contrast to organic polyenes, the difference in π orbital energy between phosphorus and nitrogen is critical to determining the onset of saturation and the magnitude of the hyperpolarizability. Conformation effects are smaller than those seen in polyenes.