Samples of soils made homoionic with K, Na, or Ca were reacted with solutions containing Cs ions, and the quantities of Cs sorbed and the rates of exchange were measured. The samples were then reacted with solutions containing K, Na, or Ca, and the quantities of Cs desorbed and the rates of exchange were again measured. Samples made homoionic with Na had a greater ion exchange capacity than samples made homoionic with K, and, in both cases, the ion exchange capacity increased with the organic matter content of the soil. For samples pretreated with Ca, the ion exchange capacity is not related in a simple way to the organic matter content. The kinetics were assessed by plotting the rate of exchange vs. the time and vs. the quantity exchanged. A first-order equation was obeyed during most of the run in Cs desorption experiments and during a limited part of the run in Cs adsorption experiments. An increase in the rate of Cs exchange was observed at the beginning of the experiments especially for Cs adsorption. This increase is presumably due to an increase of the ionic strength of the liquid phase during the exchange process.