Extending our previous investigation of dehydrated, Cs-exchanged MFI zeolites (J. Phys. Chem. B 2006, 110, 97-106) to hydrated analogues, we have determined the crystal structures of members of the Cs6.6H0.3-MFI, (.)xH(2)O series, for 0 < x < 28, from synchrotron-radiation powder diffraction data. In the fully hydrated phase, three independent Cs+ cations and six water molecules are identified in difference Fourier maps. The populations of the cations amount to 2.79/3.40/0.41 Cs/unit cell (uc) for the Cs1/Cs2/Cs3 sites, respectively, and those of the water molecules to 4/4/4/4/8/4 H2O/uc for the Ow1/Ow2/Ow3/Ow4/Ow5/Ow6 sites, respectively. Close to water saturation, the Cs3 and Ow6 sites are near each other (similar to 1.44 angstrom) and are not occupied simultaneously. At saturation, Cs cations and water molecules form three interconnected Cs(H2O) n clusters and one (H2O)(4) cluster in the MFI channel system: Cs2(H2O)(5) centered at x/y/z similar to-0.018/0.146/0.546 (midway between the intersection and the straight channels), Cs1(H2O)(4) centered at similar to 0.056/0.240/0.889 (the zigzag channel openings), Cs3(H2O)(2) centered at similar to 0.228/0.25/0.899 (in the zigzag channel), and the (H2O)(4) cluster (in the zigzag channel) bonded to Cs1 and Ow1. (H2O)(4) and Cs3(H2O)(2) exclude each other. The Cs2(H2O)(5) clusters are connected through weak Ow5(...)Ow5' hydrogen bonds (2.88 angstrom) and form polymeric chains in the straight channel direction ( 010). During progressive hydration this Cs2 cation enlarges its hydration shell, stepwise, from Cs2(H2O)(2) to Cs2(H2O)(3), to Cs2(H2O)(4), and finally to a Cs2(H2O)(5) cluster. During the dehydration process, these extraframework species migrate, and it is shown that for varying total H2O/uc loadings the individual populations of the Cs+ cations and H2O molecules strongly depend on experimental and measurement ( in situ vs ex situ) conditions. The shapes of the channels change also; except for T > 150 degrees C, in all the Cs(6.6)H(0.3)MFI(.)xH(2)O phases, the straight channel D10R (double 10-ring) pore openings (1.16 < epsilon < 1.23) become strongly elliptical. The framework structure of all the investigated phases conforms to orthorhombic Pnma space group symmetry. Hydration and dehydration in Cs6.6MFI are fully reversible processes. From a knowledge of the Cs+ locations, we are able to estimate, by computer simulations, the positions of H2O molecules in Cs(6.6)H(0.3)MFI(.)28H(2)O. The maximum theoretically possible water loading in an hypothetical and idealized cationless [Cs6.6H0.3] MFI structure amounts to 48 H2O/uc (nine independent water species), which is in fair agreement with existing high-pressure data (47 H2O/uc). This value is to be compared with the water saturation capacity obtained in a structural refinement of sealed-tube diffraction data of a proton-exchanged H(6.9)MFI(.)38H(2)O (seven independent water molecules). In the crystal structure of this H-ZSM-5 phase, the straight channel openings are almost circular (epsilon = 1.08). From this we conclude tha the main factor responsible for the flexibility of the MFI framework is the presence of the Cs(H2O) n clusters residing in, or close to, the straight channel double 10-rings.