Black, needle-shaped crystals of K₆[Sb₁₂O₁₈](SbOTe₃)₂·nH₂O (n ≈ 5) were grown in high yield from an alkaline medium under hydrothermal conditions. Microporous, single-walled [Sb₁₂O₁₈] nanotubes dominate the hexagonal crystal structure of the cetineite-type. Antimony(III) atoms form the outer and oxygen atoms the inner wall of the nanotube, making it radially dipolar, similar to a cylindrical capacitor. The inner wall is decorated with potassium cations, and water molecules fill the remaining internal volume of the nanotube. Stacks of the hitherto unknown oxidotritelluridoantimonate(V) anion, (SbOTe₃)³⁻, fill the trigonal channels in the packing of the nanotubes. As with other modifications of Sb₂O₃, the filled bonding and empty antibonding states of the nanotubes are separated by a wide band gap of about 3.6 eV. In this energy gap, the molecular orbitals of the (SbOTe₃)³⁻ anions are located. Electronic excitation involves a charge transfer from the anions to the nanotubes, which are thereby heavily n-doped and exhibit semiconducting behavior. Modification of the reaction conditions results in the crystallization of the tellurium-rich cetineite K₆[Sb₁₂O₁₈](Te₃)₂(Te₂)₃·5H₂O or the cetineite K₁₈[Sb₃₆O₅₄](SbOTe₃)₂(Te₂)₆·22H₂O, which contains [Sb₃₆O₅₄] nanotubes with an even wider diameter.