The extraction equilibria of Th in the tri-dodecylamine (TDA) and N-Benzyl-didodecylamine (BDDA) ∼ HNO 3 systems have been investigated. The influence of the amine concentration and the concentrations of nitric acid and various salting agents in the aqueous phase on the distribution ratios for Th, D Th, have been examined. The slope, of the log-log plot, of D Th vs the amine concentration is found to be close to 2 at lower concentrations of BDDA, but changes to 1.6 at higher concentration. The distribution ratio as a function of the nitric acid concentration shows a maximum value, and an equation is deduced from the extraction equilibrium for the Th and HNO 3 to estimate the maximum value; the value so calculated agrees with the experimental value. In addition, the mean activity coefficients of Th(NO 3) 4 in NaNO 3, Ca(NO 3) 2, Al(NO 3) 3 and nitric acid solutions have been calculated from the extraction data. The effect of temperature on the distribution ratio has been examined in the range 280–308 K. Plots of log D Th vs ( 1 T ) appear to be linear and values for the enthalpy of extraction are given at various amine concentrations.