The synthesis process and crystal structure evolution for a family of stoichiometric layered rare-earth hydroxides with general formula Ln₈(OH)₂₀Cl₄·nH₂O (Ln = Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Y; n ≈ 6−7) are described. Synthesis was accomplished through homogeneous precipitation of LnCl₃·xH₂O with hexamethylenetetramine to yield a single-phase product for Sm−Er and Y. Some minor coexisting phases were observed for Nd³⁺ and Tm³⁺, indicating a size limit for this layered series. Light lanthanides (Nd, Sm, Eu) crystallized into rectangular platelets, whereas platelets of heavy lanthanides from Gd tended to be of quasi-hexagonal morphology. Rietveld profile analysis revealed that all phases were isostructural in an orthorhombic layered structure featuring a positively charged layer, [Ln₈(OH)₂₀(H₂O)_n]⁴⁺, and interlayer charge-balancing Cl⁻ ions. In-plane lattice parameters a and b decreased nearly linearly with a decrease in the rare-earth cation size. The interlamellar distance, c, was almost constant (∼8.70 Å) for rare-earth elements Nd³⁺, Sm³⁺, and Eu³⁺, but it suddenly decreased to ∼8.45 Å for Tb³⁺, Dy³⁺, Ho³⁺, and Er³⁺, which can be ascribed to two different degrees of hydration. Nd³⁺ typically adopted a phase with high hydration, whereas a low-hydration phase was preferred for Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, and Tm³⁺. Sm³⁺, Eu³⁺, and Gd³⁺ samples were sensitive to humidity conditions because high- and low-hydration phases were interconvertible at a critical humidity of 10%, 20%, and 50%, respectively, as supported by both X-ray diffraction and gravimetry as a function of the relative humidity. In the phase conversion process, interlayer expansion or contraction of ∼0.2 Å also occurred as a possible consequence of absorption/desorption of H₂O molecules. The hydration difference was also evidenced by refinement results. The number of coordinated water molecules per formula weight, n, changed from 6.6 for the high-hydration Gd sample to 6.0 for the low-hydration Gd sample. Also, the hydration number usually decreased with increasing atomic number; e.g., n = 7.4, 6.3, 7.2, and 6.6 for high-hydration Nd, Sm, Eu, and Gd, and n = 6.0, 5.8, 5.6, 5.4, and 4.9 for low-hydration Gd, Tb, Dy, Ho, and Er. The variation in the average Ln−O bond length with decreasing size of the lanthanide ions is also discussed. This family of layered lanthanide compounds highlights a novel chemistry of interplay between crystal structure stability and coordination geometry with water molecules.