The mixed-actinide uranium(IV)-plutonium(III) oxalate single crystals (NH4)(0.5)[(Pu0.5U0.5IV)-U-III(C2O4)(2)center dot H2O]center dot nH(2)O (1) and (NH4)(2.7)(Pu0.7U1.3IV)-U-III(C2O4)center dot nH(2)O (2) have been prepared by the diffusion of different ions through membranes separating compartments of a triple cell. UV-vis, Raman, and thermal ionization mass spectrometry analyses demonstrate the presence of both uranium and plutonium metal cations with conservation of the initial oxidation state, U-IV and Pu-III, and the formation of mixed-valence, mixed-actinide oxalate compounds. The structure of 1 and an average structure of 2 were determined by single-crystal X-ray diffraction and were solved by direct methods and Fourier difference techniques. Compounds 1 and 2 are the first mixed uranium(IV)- plutonium(III) compounds to be structurally characterized by single-crystal X-ray diffraction. The structure of 1, space group P4/n, a = 8.8558(3) angstrom, b = 7.8963(2) angstrom, z = 2, consists of layers formed by four-membered rings of the two actinide metals occupying the same crystallographic site connected through oxalate ions. The actinide atoms are nine-coordinated by oxygen atoms from four bidentate oxalate ligands and one water molecule, which alternates up and down the layer. The single-charged cations and nonbonded water molecules are disordered in the same crystallographic site. For compound 2, an average structure has been determined in space group P6/mmm with a = 11.158(2) angstrom and c = 6.400(1) angstrom. The honeycomb-like framework [(Pu0.7U1.3IV)-U-III(C2O4)(5)](2.7-) results from a three-dimensional arrangement of mixed (U0.65Pu0.35)O-10 polyhedra connected by five bis-bidentate mu(2)-oxalate ions in a trigonal-bipyramidal configuration.