Distribution coefficients and isotherms for the adsorption of radioiodide, caesium and strontium ions ( 125 I (super -) , 134 Cs (super +) and 85 Sr (super 2+) ) were determined in untreated vermiculite and in vermiculite treated with the chloride salt of hexadecylpyridinium (HDPy (super +) ). Experimental data reveal that some of the alkylammonium ions were incorporated as HDPyCl ion pairs into the vermiculite. The fraction of ion pairs adsorbed is reflected by an increase in distribution coefficients with increasing saturation levels. Consequently, a considerable part of the iodide adsorption could be attributed to an anion exchange process. At smaller amounts of Cs (super +) and Sr (super 2+) adsorbed, distribution coefficients of the modified clay mineral are significantly reduced in comparison with the original material. These differences are less pronounced when greater amounts of cations are adsorbed. Generally, larger distribution coefficients were found for the Cs (super +) compared with the Sr (super 2+) ion in the untreated and modified samples.