The system tert-butanol-water has been confirmed by X-ray powder diffraction and singlecrystal structure analysis to contain two hydrates, a dihydrate and a higher hydrate. The composition of the latter, contradictory in the literature, could be resolved in favor of the new one of a heptahydrate. The dihydrate (m.p. 0.7°C) is monoclinic with space group P21 and Z = 2 formula units per unit cell of dimensions a = 6.017, b = 6.055, c = 10.377 Å and β = 106.60° at -150°C. The heptahydrate (m.p. -6°C, dec.) is orthorhombic with space group Pnma, Z = 4, a = 12.589, b = 15.251 and c = 6.645 Å at -150°C. Both hydrates contain characteristic layers of hydrogen-bonded O atoms, which in the heptahydrate are further linked into a three-dimensional semi-clathrate structure