The reduction of U(VI) is crucial since it greatly affects the chemical behavior of uranium in the nuclear fuel cycle and in the environment.Phosphonate ligands, on the other hand, show strong complexation towards uranium at different oxidation states and are widely used in the nuclear fuel reprocessing. Therefore,in this work, the reduction behavior of U(VI) with the presence of a phosphonate ligand is investigatedunder mild solvothermal conditions. By adjusting thereaction time,heatingtemperature, andthe counter ion species, aseriesof uraniumdiphosphonatesincluding twoU(VI),sevenU(IV),twomixed-valentU(IV/VI), and one distinct U(IV/V/VI) compounds were obtained.All these compounds were characterized by single crystal X-ray diffraction and UV-vis-NIR absorption and fluorescence spectroscopy. The structural diversityamong those compounds not only illustrates the intrinsic structural complexity in this system, but also illuminatesthein situreduction pathways that are affected by the variation of reaction conditions.TheUV-vis-NIRabsorption spectra ofthetetravalent uranium compounds show that the absorptionfeaturesareclosely related to thelocal coordination environments of the uranium centers as well asthebondingmodesof the phosphonate ligand.Thefluorescence spectraof mixed-valent uranium compounds show unique emission features of U(VI) luminescence that is partially quenched by the multiple electronic transitions of U(IV) centers in the visible and NIR regions.