Mongolian clinoptilolite-rich tuffs were examined for their ability to take up 137-Cs and 90-Sr/90-Y in the presence of competing cations (Na+, K+, Cs+, Ca2+, Mg2+, Sr2+), as nitrate salts, in the concentration range 10−1–10−3 M. Experiments were carried out by the batch technique and expressed as distribution coefficients. Clear differences were seen in Sr and Cs uptakes and those for Sr did not follow the trends expected from simple ion exchange theory. Results are discussed in relationship to the Si/Al framework compositions and cation contents of the zeolites, and suggestions made for the anomalies observed. It is concluded that the presence of some cations, potentially present in aqueous nuclear wastes, cause effects which can affect the use of clinoptilolite in the treatment of these wastes.