The hydrothermal reactions of both PuO22+ and PuO2 with phosphonates results in the formation of Pu(IV) phosphonates. Pu(CH3PO3)(2), Pu[CH2(PO3)(2)](H2O), and UO2PU(H2O)(2)[CH2(PO3)(PO3H)](2) have been isolated from these reactions and structurally characterized. Pu(CH3PO3)(2) contains six-coordinate Pu(IV) and adopts a structure closely related to that of alpha-Zr(HPO4)(2). Pu[CH2(PO3)(2)](H2O) forms a novel three-dimensional network with seven-coordinate Pu(IV) and chelating/bridging [CH2(PO3)(2)](4-) anions. The heterobimetallic U(VI)/Pu(IV) diphosphonate, UO2Pu(H2O)(2)[CH2(PO3)(PO3H)](2), also forms a three-dimensional network. To complete the An[CH2(PO3)(2)](H2O)(n) (An = Th, U, Np, Pu; n = 1, 2) and UO(2)An(H2O)(2)[CH2(PO3)(PO3H)](2) series, Th[CH2(PO3)(2)](H2O)(2) and UO2Th(H2O)(2)[CH2(PO3)(PO3H)](2) have also been prepared. These compounds are isostructural with their Np(IV) analogues.