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We describe the general synthesis of submicrometer-thick rare-earth/lanthanide sesquioxide (RE2O3) films with tailorable pore and grain sizes via polymer templating of hydrated chloride salt precursors. Mesostructured RE2O3 (RE = Sm, Tb-Lu) ceramics with cubic pore symmetry and high surface area (SBET ≥ 50 m² g-1) were prepared using different diblock copolymer structure-directing agents and were characterized by a combination of electron microscopy, in situ and ex situ grazing incidence small-angle X-ray scattering, N2 physisorption, X-ray photoelectron spectroscopy, X-ray diffraction including Rietveld refinement, and ultraviolet-visible spectroscopy. In the present work, we specifically focus on Dy2O3 and Yb2O3 and use both of these materials as model systems to study, among other things, the film formation and microstructure. Our research data collectively demonstrate that (1) record pore sizes of up to 42 nm in diameter can be achieved without the need for swelling agents, (2) the nanostructure can be preserved up to 1000 °C for the heavier oxides, (3) the sizes of the optical band gaps (4.9-5.6 eV) are comparable to those reported for single crystals, (4) the sol-gel-derived materials are single phase and adopt the C-type crystal structure, and (5) the grain growth is virtually linear, with domain sizes in the range of 3-16 nm. We also show that, except for Yb2O3, all of the samples have a fiber texture and the preferred orientation is significant in Sm2O3 and Lu2O3 films (March parameter G2 < 0.1). Overall, the synthesis parameters described in this work provide a blueprint for the preparation of thermally stable rare-earth oxide ceramics with both a mesoporous morphology and iso-oriented nanocrystalline walls. [ABSTRACT FROM AUTHOR]