The exchange of Cs+ into H1.22K0.84ZrSi3O9 center dot 2.16H(2)O (umbite-(HK)) was followed in situ using time-resolved X-ray diffraction at the National Synchrotron Light Source. The umbite framework (space group P2(1)/c with cell dimensions of a = 7.2814(3) angstrom, b = 10.4201(4) angstrom, c = 13.4529(7) angstrom, and beta = 90.53(1)degrees) consists of wollastonite-like silicate chains linked by isolated zirconia octahedra. Within umbite-(HK) there are two unique ion exchange sites in the tunnels running parallel to the a-axis. Exchange Site 1 is marked by 8 member-ring (MR) windows in the bc-plane and contains K+ cations. Exchange Site 2 is marked by a larger 8-MR channel parallel to [100], and contains H2O molecules. The occupancy of the Cs+ cations through these channels was modeled by Rietveld structure refinements of the diffraction data and demonstrated that there is a two-step exchange process. The incoming Cs+ ions populated the larger 8-MR channel (Exchange Site 2) first and then migrated into the smaller 8-MR channel. During the exchange process a structural change occurs, transforming the exchanger from monoclinic P2(1)/c to orthorhombic P2(1)2(1)2(1). This structural change occurs when Cs+ occupancy in the small cavity becomes greater than 0.50. The final in situ ion exchange diffraction pattern was refined to yield umbite-(CsK) with the molecular formula H0.18K0.45Cs1.37ZrSi3O9 center dot 0.98H(2)O and possessing an orthorhombic unit cell with dimensions a = 10.6668(8) angstrom, b = 13.5821(11) angstrom, c = 7.3946(6) angstrom. Solid state Cs-133 MAS NMR showed there is only a slight difference between the two cavities electronically. Valence bond sums for the completely occupied Exchange Site 1 demonstrate that Cs-O bonds of up to 3.8 angstrom contribute to the coordination of the Cs+ cation.