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The oxygen, nitrogen and fluorine core binding energies of ONF 3 have been redetermined. The data indicate strong hyperconjugation, with N-O π - bonding and weak N-F σ-bonding. The core and valence ionization potentials together indicate that the HOMO of ONF 3 (derived mainly from the O p π orbitals) is weakly antibonding. This result is consistent with other data if it is assumed that the latter orbital has considerable fluorine lone pair character and that the orbital derived mainly from the N-F σ orbitals (at lower energy) has considerable N-O π character. The core and valence ionization potentials of OPF 3 and OPCl 3 indicate that the oxygen “lone pair” orbitals of these molecules are at lower energies than the corresponding hypothetical nonbonding oxygen p orbitals. These results imply significant participation of phosphorus 3dπ orbitals in the P-O bonds.