The details of the ion-exchange properties of the layered sulfide material K2xMnxSn3-xS6 (x = 0.5-0.95) (KMS-1) with Cs+ and Rb+ cations are reported. X-ray photoelectron spectroscopy (XPS), elemental analyses, and powder and single-crystal diffraction studies revealed that the Cs+ and Rb+ ion exchange of KMS-1 is complete (quantitative replacement of K+ ions) and topotactic. These data also revealed that the Cs+ exchange is accompanied with a rare topotactic oxidation of Mn2+ to Mn3+ caused by atmospheric oxygen, while the Rb+ ion exchange only slightly alters the oxidation state of the layer manganese atoms. The absorption of Cs+ by KMS-1 follows the Langmuir model with a high exchange capacity of 226(4) mg/g (pH approximate to 7) and distribution coefficients as high as 2 x 10(4) mL/g. KMS-1 displays significant cesium uptake both under strongly acidic (pH 0.7-2.6) or basic conditions (pH 10-12). The kinetics of Cs+ capture by KMS-1 is fast (>90% removal of similar to 1 ppm of Cs+ within only 5 min). KMS-1 was also found capable to efficiently absorb Cs+ from complex solutions containing various competitive cations in large excess. KMS-1 (containing Mn3+ ions) can be regenerated and reused for Cs+ exchange with an exchange capacity very similar to that of the pristine KMS-1. The results indicate that layered metal sulfides with ion-exchange properties may be considered as highly selective and cost-effective sorbents for remediation of water contaminated with the radioactive Cs-137 isotope. The selectivity over other alkali ions for Cs originates not from a size effect but from the more favorable Cs center dot center dot center dot S soft Lewis acid/Lewis base interactions.