[en] The crystalline phases of CsOH and CsOD have been investigated between 23 K and the melting point by X-ray and neutron diffraction experiments as well as by calorimetric, IR, and Raman spectroscopic measurements. Crystal data are presented. In the modification stable at low temperatures which resembles a TlI-type like structure the charge-asymmetric hydroxide ions are onedimensional hydrogen bonded. Certain forms of movements of the anions occur by warning up. In the high temperature NaCl-type structure the anions rotate quasi freely. The results of the various methods of investigation of the alkali metal hydroxides in the solid state are discussed with respect to the chemical bonding in these compounds. (author)