Ion exchange isotherms for the sodium form of birnessite, a layered manganese oxide common in soils, were determined using isotope dilution analysis, by employing 22Na to pre-label the solid. The results were represented by plotting the equivalent ionic fraction of an entering cation in the ion exchanger versus the equivalent ionic fraction of the entering cation remaining in solution. Studying these ion exchange isotherms for sodium-univalent (i.e. NH4–Na, Li–Na, K–Na, and Cs–Na) and sodium-divalent pairs (Mg–Na, Ca–Na, Sr–Na and Ba–Na) on 22Na-birnessite made it possible to obtain a clear picture of the exchangeable sites available in birnessite. The isotherms also gave an estimation of the effect of cations likely to be present in aqueous environments on the uptake of the fission product radioisotopes of caesium and strontium onto soils.