Abstract Three different solvates of TBA 6[Nb 10O 28] (TBA = tetra- n-butylammonium) were structurally characterized. The results revealed that two water molecules are hydrogen-bonded to the terminal oxygens of the [Nb 10O 28] 6− anion in the same manner in all of the solvates. Decaniobate [Nb 10O 28] 6− dimerizes by the action of HCl to form icosaniobate [Nb 20O 54] 8−, while icosaniobate breaks up into decaniobate [Nb 10O 28] 6− by the action of TBAOH. Decaniobate also dimerizes spontaneously to form icosaniobate [Nb 20O 54] 8− in CH 2Cl 2 even if no acid is added to the solution. The reaction was followed by IR spectroscopy, and the results suggested the reaction is second order with respect to the concentration of [Nb 10O 28] 6−.