AbstractSorption experiments have been performed by a batch method, to study the effects of humic acid of different molecular size on the complexing stability with60Co,85Sr,137Cs,237Np and241Am, and on the sorption behavior of these radionuclides on a sandy soil.Equilibrium constantsKin the sorption of137Cs and237Np onto the soil were not changed at different concentrations of humic acid since137Cs and237Np do not interact with humic acid, while those of60Co and241Am decreased with increasing humic acid concentration due to forming humic complexes. However, theKof85Sr was not changed at different humic acid concentrations, despite85Sr interacts with humic acid. This contradiction was probably caused from that a main binding of85Sr with humic acid is not based on coordination bond but electrostatic force, due to relatively high concentration of non–radioactive strontium. The theoretical sorption model taking account of the interaction of60Co and241Am with humic acid could well reproduce the values ofKfor each radionuclide at different concentrations of humic acid.Concentration profiles of the radionuclides in each size fraction of the solution before and after the sorption experiments were examined by ultrafiltration technique. The reduction of concentration of60Co in the fraction less than 300,000 of cutoff molecular weight (MW) and that of concentration of241Am in the fraction larger than 100,000MW, respectively, by the sorption onto the soil decreased with increasing humic acid concentration. This decrease resulted in the decrease in theKof60Co and241Am with increasing humic acid concentration.