Abstract: This study presents an overview of our post-Chernobyl investigations of the mobility of radiocaesium in sediments, as expressed in terms of its sediment/water distribution coefficients or Kd values, and focuses on the two major factors controlling radiocaesium mobility: the quantity of selective binding sites and the competition for these sites by other cations. In situ Kd values have been measured in a number of very different western European lake sediments and show a variation of over three orders of magnitude. This variability can be quantitatively interpreted in terms of a competitive ion-exchange process between radiocaesium and ammonium ions on highly selective binding sites at the frayed particle edges of illitic clay minerals. These findings allow the radiocaesium Kd in freshwater sediments to be predicted, within acceptable limits, on the basis of the quantity of highly selective exchange sites and the pore-water NH4 concentration. The implications of these findings for the exchangeability of sediment-bound radiocaesium are discussed.