Potassium dioxodiaquaperoxyoxalatouranate(VI) was obtained by reaction of uranyl nitrate with oxalic acid and then hydrogen peroxide in the presence of potassium ion. The complex was subjected to chemical analysis. The thermal decomposition behavior of the complex was studied using TG, DTA and DTG techniques. The solid complex salt and the intermediate product of its thermal decomposition were characterized using IR absorption and X-ray diffraction spectra. Based on the data from these physicochemical investigations, the structural formula of the complex was proposed as K-2[UO2(O-2)C2O4(H2O)(2)]. (C) 2003 Published by Elsevier Science B.V.