The unsymmetrical diamide methylbuthylmalonamide has been synthesized and used in the extraction of U(VI), Pu(IV) and Am(III) in benzene medium. The distribution ratio for the three cations was found to increase with increasing aqueous nitric acid concentration. U(VI) and Pu(IV) were found to be extracted as disolvates while Am(III) as a trisolvate. The thermodynamic parameters determined by the temperature variation method showed the extraction reactions to be mainly enthalpy-controlled. Am(III) was found to be back-extracted with dilute nitric acid, while Pu(IV) by dilute nitric acid-hydrofluoric acid mixture and U(VI) by dilute sodium carbonate solution.