If the full scientific and technological potential of mesostructured materials is to be achieved, systems with continuous domains in the form of single crystals or films must be prepared. Here we report a reliable and facile system for making large single-crystal particles of chalcogenido mesostructured materials with a highly organized cubic structure, accessible pore structure, and semiconducting properties. Building blocks with square planar bonding topology, Pt2+ and [Sn2Se6](4-), in combination with long-chain pyridinium surfactants (CnPyBr, n = 18, 20) favor faceted single-crystal particles with the highest possible space group symmetry la (3) over bard This is an important step toward developing large single-domain crystalline mesostructured semiconductors and usable natural self-assembled antidot array systems. The tendency toward cubic symmetry is so strong that the materials assemble readily under experimental conditions that can tolerate considerable variation and form micrometer-sized rhombic dodecahedral cubosome particles. The c-CnPyPtSnSe materials are the first to exhibit reversible ion-exchange properties. The surfactant molecules can be ion-exchanged reversibly and without loss of the cubic structure and particle morphology. The cubosomes possess a three-dimensional open Pt-Sn-Se framework with a low-energy band gap of similar to1.7 eV.