Solid state reactions of UO2, ThO2, PuO2 and their mixed oxides (U,Th)O2 and (U,Pu)O2 were carried out with sodium nitrate upto 900° C, to study the formation of various phases at different temperatures, which are amenable for easy dissolution and separation of the actinide elements in dilute acid. Products formed by reacting unsintered as well as sintered UO2 with NaNO3 above 500°C were readily soluble in 2M HNO3, whereas ThO2 and PuO2 did not react with NaNO3 to form any soluble products. Thus reactions of mixed oxides (U,Th)O2 and (U,Pu)O2 with NaNO3 were carried out to study the quantitative separation of U from (U,Th)O2 and (U,Pu)O2. X-ray diffraction, X-ray fluorescence, thermal analysis and chemical analysis techniques were used for the characterization of the products formed during the reactions.