[en] The exchange of Cs⁺ into H_1.22K_0.84ZrSi₃O₉ · 2.16H₂O (umbite-(HK)) was followed in situ using time-resolved X-ray diffraction at the National Synchrotron Light Source. The umbite framework (space group P2₁/c with cell dimensions of a = 7.2814(3) (angstrom), b = 10.4201(4) (angstrom), c = 13.4529(7) (angstrom), and β = 90.53(1)^o) 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 P2₁/c to orthorhombic P2₁2₁2₁. 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 H_0.18K_0.45Cs_1.37ZrSi₃O₉ · 0.98H₂O 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 ¹³³Cs 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.