The crystal structures of the title complexes, [NpO₂(OPPh₃)₂(NO₃)₂](1), [UO₂(OPPh₃)₂(NO₃)₂](2), and [NpO₂(OPPh₃)₂Cl₂](3), have been determined using X-ray diffraction methods. Compounds (1) and (2) are isomorphous and isostructural. Their crystals are monoclinic, space group P2₁/c, with hexagonal-bipyramidal co-ordination about the heavy atoms. The bidentate nitrate groups are trans to each other and bonded to the metal (M–O 2.52–2.54 Å), as are the triphenylphosphine oxide ligands (M–O 2.35–2.36 Å). The M–O bond lengths in the MO₂²⁺ cations (M = Np or U) are 1.739(10) and 1.764(9)Å respectively. Compound (3) is monoclinic, space group P2₁, and exhibits distorted octahedral co-ordination about neptunium, the trans ligands having Np–Cl lengths of 2.622(14) and 2.645(13)Å; for the triphenylphosphine oxide groups, Np–O lengths are 2.261(19) and 2.288(15)Å. The NpO₂²⁺ cation has Np–O bond lengths of 1.721(16) and 1.751(18)Å. Lattice parameters are: for (1)a= 11.038(5), b= 18.838(7), c= 10.897(4)Å, β= 128.27(3)°, and z= 2; for (2), a= 11.043(1), b= 18.880(3), c= 10.870(2)Å, β= 128.17(1)°, and Z= 2; for (3), a= 10.757(4), b= 18.716(4), c= 10.734(3)Å, β= 124.05(2)°, and Z= 2. The structures have been refined to R values of 0.049 (1), 0.028 (2), and 0.042 (3) using respectively, 2 821, 2 638, and 2 063 observed diffractometer-measured intensities.