In this work, the formation of uranium species and their stabilities in Na-U(VI)-CO(3)-OH-H(2)O(2) solutions at different pHs are studied by Raman spectroscopy. In this solution, the UO(2)(O(2))(CO(3))(2)(4-) species was formed together with three other uranium species of UO(2)(O(2))(2)(2-), UO(2)(CO(3))(3)(4-), and a speculated uranium species of the uranyl carbonate hydroxide complex, UO(2)(CO(3))(x)(OH)(y)(2-2x-y), which showed remarkable Raman peaks at approximately 769, 848, 811, and 727 cm(-1), respectively. The UO(2)(O(2))(CO(3))(2)(4-) species disappeared at pH conditions where bicarbonate dominated, and its Raman peak could be clearly observed in only a narrow pH range from approximately 9 to 12. When the pH of the solution increased further, the UO(2)(O(2))(CO(3))(2)(4-) species changed to UO(2)(CO(3))(3)(4-) and the UO(2)(CO(3))(x)(OH)(y)(2-2x-y) species. Moreover, the UO(2)(O(2))(CO(3))(2)(4-) species continuously decomposed into uranyl tricarbonate in the carbonate solution at an elevated temperature because of the instability of the peroxide ion, O(2)(2-), in alkaline conditions.