UO₂, ThO₂ and (U_xTh_1-x)O₂ (x = 0.1 and 0.5) were reacted with NH₄NO₃ in different molar ratios up to 400 °C in air, to study the dissolution behavior. Solid state reaction of UO₂ with different moles of NH₄NO₃ showed various intermediate products, depending on the amount of NH₄NO₃. The end products at 400 °C were identified as U₃O₈ and UO₃, respectively when 1 and 1.5 mol of NH₄NO₃ were used. When UO₂ and NH₄NO₃ were reacted in 1:4 molar ratio, the intermediate products identified were (NH₄)₂UO₂(NO₃)₄ and NH₄UO₂(NO₃)₃ at 180 and 220 °C, respectively. XRD data of both the compounds were indexed on orthorhombic system. ThO₂ did not show any reaction with NH₄NO₃ up to 400 °C. The reaction of (U,Th)O₂ with 4 mol of NH₄NO₃ was studied to see the dissolution and separation of U from the mixed oxide. X-ray diffraction, thermal analysis and chemical analysis techniques were used for the characterization of the products.