Sorption of Cs by a synthetic cation‐exchange resin and by several clay minerals, over a range of low Cs concentrations, was studied in the presence of different counter‐ions. An ionexchange equation was used to predict the sorption of Cs from dilute solutions. Exchange isotherms showed that the sorption of Cs by the resin followed these predictions: Exchange isotherms were straight lines with unit slope. Sorption of Cs by clay minerals, however, did not follow these predictions except when K+ was the competing cation. When Ca2+ and Al3+ were the competing cations, the exchange isotherms for montmorillonite were straight lines with less than unit slope, showing that a larger proportion of Cs was sorbed with decreased Cs concentrations. The exchange isotherms of illite and vermiculite, on the other hand, had at least two distinct slopes suggesting that more than one type of exchange site is involved in the sorption of Cs by these minerals. These conclusions were also supported by estimates of the equilibrium constants calculated from Vanselow's equation.