The effects of ionic strength on the sorption of Eu(III) on kaolinite in the presence of humic acid (HA) were examined using a batch method. Eu(III) sorption increased with increased ionic strengths in the pH range of 6 to 10, but was insensitive to ionic strength in the pH range below 5. Adsorption of HA onto kaolinite also increased in the pH range of 6 to 10, indicating that the increase in adsorption of Eu(III)-humate complexes is a predominant process for the increase in Eu(III) sorption.