Adsorption of thorium onto nitric acid modified multi-walled carbon nanotubes was investigated by central composite design as a function of contact time, pH, initial thorium concentration and temperature. The results showed that optimum uptake capacity was 65.75 +/- 2.23 mg.g(-1) with respect to pH = 4, initial thorium concentration of 100 mg.L-1, 25 degrees C and 15 min contact time. Thermodynamic parameters [standard enthalpy (Delta H degrees), entropy (Delta S degrees), and free energy (Delta G degrees)] were calculated, and the results indicated that adsorption was endothermic. Langmuir, Freundlich and Dubinin-Radushkevich isotherms have been investigated in order to characterize the adsorption process in the range of 25-100 mg.L-1 initial thorium concentration. The Freundlich isotherm is the best suited as a model because it has the highest correlation coefficient (R-2 = 0.9485). The pseudo-second order kinetics well defined the adsorption process.