Studies have been made on the extraction of U(VI) from nitric acid media by three n-alkylcaprolactams (hexyl-: Bg, octyl-:Bg, dodecyl-rBn) and their mixtures in toluene. The distribution ratio of U(VI) increased in the sequence of B12> B8> B6, where the extracted species have been found to be UO2(NO3)2(B)2and UO2NO3)3B12H+by slope analysis method. Moreover, the molecular structure of the extracted complex with B8was demonstrated clearly by use of a single-crystal X-ray diffractometer. The enthalpy changes in the U(VI) extraction were evaluated by.temperature variation method, indicating that the extraction process is exothermic. Stripping of U(VI) from the organic phases was also discussed. For binary mixtures of B6, a small synergistic enhancement was found, the sequence being of B6-B12> B8-B12> B6-B8