The effect of the addition of various humic substances on the adsorption of caesium on two mineral clays has been studied. All measurements were carried out in dilute suspension under controlled conditions of temperature and ionic strength. Only a small proportion of the humic substance was adsorbed on the clays ((10%). In general, the a$nity of the clay}humic complexes for caesium was less than that of the bare clay. The decrease was greater for illite than for montmorillonite, and greatest at trace concentrations of caesium and increased with increasing concentration of each humic substance. However, no correlation was found between the amount of humic substance adsorbed and the decrease in Cs adsorption when all complexes were considered. Neither size nor the origin of the humic substances could explain the extent of the adsorption decrease. Since neither steric hindrance nor decrease in the number of adsorption sites was the driving force behind this phenomenon, it is hypothesized that the a$nity of the clay surface is modi"ed by the organic macromolecules. The highly selective frayed edge sites of illite are particularly sensitive to the adsorption of polyanions because of their proximity to anion adsorption sites. The observed decrease in Cs adsorption may contribute to the unexpectedly high bioavailability of Cs in organic soils.