Sorption characteristics of Cs+ and Sr++on montmorillonite and kaolinite type clays and soil fractions from various regions of Turkey were studied using the batch method. 137Cs and 90Sr were used as tracers. Concentrations of Cs+ and Sr++ ions ranged from 10-8to 10-2 mol/1; natural groundwater was used and the grain size of the solid particles was < 20 yμm. Equilibrium was reached in 4 - 7 days for Cs+and 7 - 11 days for Sr++. The distribution coefficient, RD, increased with decreasing grain size, suggesting mainly a surface phenomenon. The sorption isotherms were non-linear suggesting at least two different sorption processes. The sorption was found to be predominantly reversible. Cs+ was sorbed much stronger than Sr++in all samples. © 1988, Walter de Gruyter. All rights reserved.