Tetragonal caesium hydroxide monohydrate, CsOH x H(2)O, a clathrate hydrate, is a polymorph of three known hexagonal or pseudo-hexagonal modifications. It was obtained as a by-product in a high-pressure experiment. Whether it is a high-pressure polymorph, however, remains to be verified. The Cs atoms are situated in cavities of the form of a bicapped pentagonal prism, within an infinite three-dimensional hydrogen-bonded oxygen framework that is locally identical to layers found in the hexagonal modifications. The Cs atom and one of the two H atoms are at sites with (macro)4m2 symmetry, the O atom has mm site symmetry and the second H atom has 2/m symmetry