A tetra-n-butylammonium (TBA) salt of [H-4.5(Ta6O19)](3.5-) was synthesized by reacting hydrous tantalum oxide with TBAOH. X-ray structural analysis of TBA(3.5)[H-4.5(Ta6O19)]center dot 2THF center dot 5.5H(2)O (THF = tetrahydrofuran) revealed that this compound consists of a hydrogen-bonded, rod-shaped tetramer of hexatantalate that is almost 30 angstrom long together with TBA cations and solvent molecules of crystallization [a = 20.6354(5) angstrom, b = 25.5951(7) angstrom, c = 37.2058(8) angstrom, alpha = 77.092(1)degrees, beta = 86.177(1)degrees, gamma = 88.683(1)degrees, V = 19110.9(8) angstrom(3), Z = 8, and space group P (1) over bar]. H-1 NMR spectra showed that this tetrameric structure is maintained in solution.