The sorption of Cs+ on natural kaolinite has been studied using time-of-flight secondary ion mass spectrometry (TOF-SIMS). Depth profiling up to 70 Å was performed to study the change in the amount of sorbed Cs+ as a function of depth in the kaolinite matrix. Quantitative determination of the amounts of primary cations in the kaolinite structure before and after sorption of Cs+ ions was carried out to identify which cations are possibly taking part in the sorption process. The experimental results showed that large amounts of Cs+ are sorbed onto the surface of kaolinite and that sorption decreases sharply over the first 10-Å depth. The fact that kaolinite surface was negatively charged under the operating pH indicates that physisorption has an important contribution to the sorption process. The results also showed that Na+, K+, Li+, Ca2+, Mg2+ and Fe3+ were involved in the sorption process with Cs+ and that the total decrease in the amounts of these cations is close to the amount of sorbed Cs+, suggesting that ion exchange is the dominant sorption mechanism. Copyright (C) 1999 Elsevier Science B.V. All rights reserved.