Highly selective ion-exchange properties and -exchange capacities of the open framework chalcogenide material K6Sn[Zn4Sn4S17] (1) with Cs+ and NH4+ are reported. Because the structure of this framework is known in great detail, these studies are a rare example where structure/property relationships can be directly drawn. 1 possesses three types of micropore cavities. The largest pore of 1 presents an exact fit for Cs+ and exhibits high selectivity for this ion, as demonstrated by competitive ion-exchange experiments. The next largest pore has a greater capacity (up to four cations) and is well suited for NH4+ ions. This leads to a high ammonium-exchange capacity for 1 of 3.06 mequiv/gr, which is close to the NH4+-exchange capacities of natural zeolites. The single-crystal structures of ammonium-exchanged products at various stages reveal an unusual mechanism for the exchange process of 1 which involves diffusion of ammonium cations from the large cavity to the small ones of the framework. Thermal analysis of one of these ammonium-exchanged products, in combination with mass spectroscopy, showed the decomposition of NH4+ cations to NH3 and H2S with the parallel transformation of the exchanged product to a mixture of crystalline phases. Since K6Sn[Zn4Sn4S17] can be grown in suitably large crystals ( much larger than most zeolites), it defines an excellent model system in which ion-exchange processes and products can be characterized and studied in detail in various reaction stages.