Two tetravalent uranium compounds have been characterized. The structure of a new uranium(IV) phosphonate, U(O3PC6H5)2, has been solved from laboratory X-ray powder diffraction data by usingab initiomethodology. U(O3PC6H5)2crystallizes in the space groupC2/mwitha= 9.4559(7) Å,b= 5.6769(5) Å,c= 14.9687(12) Å, β = 96.539(5) ,V= 798.3(1) Å3,Z= 2. The reliability factors wereRWP= 8.0%,RP= 6.04%, andRF= 3.0%. The structure is lamellar, and the framework of the U(O3P)2layers is similar to that of the α-Zr(HPO4)2·H2O-type structure, although the symmetry of the phosphonate group is higher than that of the phosphate groups in α-Zr(HPO4)2·H2O and the phosphonate group in Zr(O3PC6H5)2. The phenyl groups are located in the interlamellar space, being inclined 10° to thec-axis. The phenyl rings are tilted out 53° from theacplane, and they are disordered. We have also characterized this compound by UV-VIS-IR spectroscopies and thermal analysis. The thermal decomposition product is uranium(IV) pyro phosphate. This compound was identified through its X-ray powder diffraction pattern. UP2O7crystallizes in thePa3space group (a= 8.6311(2) Å,V= 642.99(4) Å3,Z= 4). The structure belongs to the cubic ZrP2O7-type structure. The reliability factors wereRWP= 11.7%,RP= 8.6%, andRF= 10.4%. Disorder has been found in the oxygen that bridges the pyrophosphate groups, leading to an angular P–O–P arrangement. The VIS-near-IR adsorption spectra revealed the uranium(IV) presence and the oxygen environment.