N,N,N′,N′-tetrabutylsuccinylamide (TBSA) was synthesized, characterised and used for the extraction of U(VI) and Th(IV) from nitric acid solutions into n-dodecane. Extraction distribution coefficients of U(VI) and Th(IV) as a function of aqueous nitric acid concentration, extractant concentration and temperature have been measured and found that n-dodecane as diluent was superior to 50% 1,2,4-trimethyl benzene (TMB) and 50% kerosene, (OK) system for extraction of U(VI) and Th(IV). The compositions of extracted species, equilibrium constants and enthalpies of extraction reactions have also been calculated. The formation of the 1∶2∶1 complex of uranyl(II) ion or the 1∶4∶1 complex of thorium(IV) ion, nitrate ion and TBSA and extracted species was further confirmed by the IR spectra of saturated extract of U(IV) and Th(IV).