The in situ hydrothermal reduction of Np(VI) to Np(IV) and Pu(VI) to Pu(IV) in the presence of 1,2-phenylenediphosphonic acid (PhP2) results in the crystallization of Np[(CH4)-H-6(PO3H)(2)](2) center dot 2H(2)O (NpPhP2) and Pu[C6H4(PO3H)-(PO3H2)][C6H4(PO3H)(PO3)] center dot 2H(2)O (PuPhP2), respectively. Similar reactions have been explored with Ce(IV) resulting in the isolation of the Ce(IV) phenylenediphosphonate Ce[C6H4(PO3H)-(PO3H2)][C6H4(PO3H)(PO3)] center dot 2H(2)O (CePhP2). Single crystal diffraction studies reveal that although all these three compounds all crystallize in the triclinic space group P (1) over bar, only PuPhP2 and CePhP2 are isotypic, whereas NpPhP2 adopts a distinct structure. In the cerium and plutonium compounds edge-sharing dimers of MO8 polyhedra are bridged by the diphosphonate ligand to create one-dimensional chains. NpPhP2 also forms chains. However, the NpO8 units are monomeric. The protonation of the ligands is also different in the two structure types. Furthermore, the NpO8 polyhedra are best described as square antiprisms (D-4d), whereas the CeO8 and PuO8 units are trigonal dodecahedra (D-2d). Bond-valence parameters of R-o = 1.972 and b = 0.538 have been derived for Np4+ using a combination of the data reported in this work with that available in crystallographic databases. The UV-vis-NIR absorption spectra of NpPhP2 and PuPhP2 are also reported and used to confirm the tetravalent oxidation states.