A series of tertiary phosphine adducts of [Nb(O)Cl3] is described. The compounds [Nb(O)Cl3L2] (L = PMe3, PMePh2, PPh3 or PEt3), [Nb(O)Cl3(Ph2PCH2CH2PPh2)] and [Nb(O)Cl3(PMe2Ph)3] have been prepared via reaction of either [{Nb(O)Cl3}n] or [Nb(O)Cl3(thf)2] (thf = tetrahydrofuran) with a stoichiometric amount of the phosphine in dichloromethane solvent. The influence of the differing ligand fields on the infrared stretching frequency of the niobium-oxygen bond is discussed in terms of the geometry of the complex and the stereoelectronic properties of the attendant ligands.