[en] N,N dialkylamides are good extractants of UO₂²⁺ ions from the nitric solutions obtained on dissolution of nuclear irradiated fuels. The extraction mechanism of U(VI) ions proved to be rather complex. Two species were identified by spectrophotometry in the organic phase: UO₂(NO₃)₂L₂ (1) and UO₂(NO₃)₃HL (2), L standing for the amide molecule. The complex (1) is typical for neutral molecules extractants. However, when log D(U(VI)) is plotted against log C (amide), the slope of the straight line obtained is greater than 2, which suggests that amide (1) interactions take place in the 2sup(nd) coordination sphere of U(VI) ions. The crystal structure of (1) where L is the N,N-di-n-butyldodecanamide show that Van der Waals interactions occur between the dodecyl chains of two (1) molecules. The complex (2) is characteristic for anionic extractants. For the amide molecules studied, it takes place in very acidic media. In order to investigate the nature of the UO₂(NO₃)₃HL complex, the infra-red shift of νc=0 vibration were compared in the following compounds: free amide, HNO₃^-L, phenol L, UO₂(NO₃)₃HL. The results are discussed. 14 refs