The objective of the study was to contribute to the understanding of the influence of the structure and the 2:1 layer dimension of smectites on cation exchange capacity (CEC) reduction and the hydration behavior of Li-saturated smectites after heating. Five montmorillonites extracted from bentonites of different provenance were saturated with Li+ and heated to 300°C. Initial montmorillonites and montmorillonites with reduced layer charge (RCM) were characterized by comprehensive mineralogical analysis supplemented by CEC measurements, surface-area measurements by Ar adsorption, and 7Li, 27Al, and 29Si magic-angle spinning nuclear magnetic resonance spectroscopy (MAS NMR). The CEC of the initial montmorillonites varied between 89 and 130 cmol(+)/kg while the CEC of the RCM prepared at 300°C varied between 8 and 25 cmol(+)/kg. The lateral dimension of the 2:1 layers varied between 70 and 200 nm. The greatest decrease in CEC was observed for the montmorillonite with the largest diameter of the 2:1 layers and the smallest decrease was observed for the montmorillonite with the smallest diameter of the 2:1 layers. 7Li MAS NMR revealed an axially symmetric chemical environment of the hydrated interlayer Li+ with ηΔ = 0 for the chemical shift anisotropy tensor for unheated montmorillonites with >33% tetrahedral layer charge (ξ). The chemical environment is typical of inner-sphere hydration complexes of interlayer Li+. An axially non-symmetric chemical environment of the interlayer Li+ with ηCS of close to one was observed for all RCM. While the remaining CEC of RCM prepared at 300°C reflected the variable CEC at the edges, and thus the lateral size or aspect ratio of the 2:1 layers, the hydration complex of interlayer Li+ was strongly determined by the isomorphic substitutions in the dioctahedral 2:1 layers.