Hydrated uranium peroxide (UO4 . nH(2)O with n = 2, 4 or 9/2) can be obtained by the reaction between hydrogen peroxide and uranyl nitrate. The optimal conditions for obtaining these compounds in a nitric medium require knowledge of the solid-liquid equilibrium of the reciprocal quaternary system H2O-UO2(NO3)(2), H2O2/(HNO3)2, UO4. The spontaneous decomposition of hydrogen peroxide into water and oxygen gas has led us to approach this problem through isoplethic isothermal sections including the H2O-H2O2 binary system. To guide the experimental study and to simplify the interpretation of results, liquidus curves of uranium peroxide tetrahydrate have been calculated on the basis of the solubility product. Two synthetic experimental methods, using pH measurements and isoplethic thermal analysis, have been selected and adapted. Furthermore, some investigations in concentrated medium have allowed us to define operating conditions leading to reproducible results.