A reinvestigation of the alkali metal thiosulfates has led to the new phase Cs2S2O3 center dot H2O. At first cesium thiosulfate monohydrate was obtained as a byproduct of the synthesis of Cs4In2S5. Further investigations were carried out using the traditional synthesis reported by J. Meyer and H. Eggeling. Cs2S2O3 center dot H2O crystallizes in transparent, colorless needles. The crystal structure of the title compound was determined by single crystal X-ray diffraction at room temperature: space group C2/m (No. 12), unit cell dimensions: a = 11.229(4), b = 5.851(2), c = 11.260(5) angstrom, beta = 95.89(2)degrees, with Z = 4 and a cell volume of V = 735.9(5)angstrom(3). The positions of all atoms including the hydrogen atoms were located in the structure refinement. Cs2S2O3 center dot H2O is isotypic with Rb2S2O3 center dot H2O. Isolated tetrahedra [S2O3](2-) are coordinated by the alkali metal cations, and in addition they serve as acceptors for hydrogen bonding. For both Cs atoms the shortest distances are observed to oxygen atoms of the S2O32- anions where as the terminating sulfur atom has its shortest contacts to the water hydrogen atoms. Thus, an extended hydrogen bonding network is formed. The title compound has also been characterized by IR spectroscopy. IR spectroscopy reveals the vibrational bands of the water molecules at 3385 cm(-1). They show a red shift in the OH stretching and bending modes as compared to free water. This is due both to the S center dot center dot center dot H hydrogen bonding and to the coordination of H2O molecules to the cesium atoms.