In order to examine the applicability of a thermomorphic ionic liquid, betainium bis(trifluoromethylsulfonyl)imide ([Hbet][Tf2N]), for the decontamination of solid wastes contaminated with uranium, we have studied fundamental extraction behavior of U(VI) from HNO3 aqueous solutions to [Hbet][Tf2N] phases using a homogeneous liquid-liquid extraction method. As a result, it was found that the U(VI) species are extracted with relatively high distribution ratios (D > 1) in low acidic condition without any additional extractant, and proposed that the U(VI) species are extracted through the exchange reaction between [Hbet](+) and [UO2(bet)](2+). The complex formation of [UO2(bet)](2+) was found to play an important role in the extraction, because the U(VI) species were not extracted with trimethylpropylammonium bis(trifluoromethylsulfonyl)imide ([TMPA][Tf2N]) without any functional groups. We also examined the recovery method of U(VI) species from the [Hbet][Tf2N] phase. As a result, it was found that the precipitation of uranyl peroxide with the addition of H2O2 aqueous solution is the useful method for recovering U(VI) species. (C) 2015 Elsevier B.V. All rights reserved.