The 2-thio-ethanesulfonate anion is the smallest known coenzyme in nature (HS-CoM) and plays a key role in methano-genesis by anaerobic archaea, as well as in the oxidation of alkenes by Gram-negative and Gram-positive eubacteria. The title compound, Na(+)·C(2)H(5)O(3)S(2) (-)·H(2)O, is the Na(+) salt of HS-CoM crystallized as the monohydrate. Six O atoms form a distorted octa-hedral coordination geometry around the Na atom, at distances in the range 2.312 (4)-2.517 (3) Å. Two O atoms of the sulfonate group, one O atom of each of three other symmetry-related sulfonate groups plus the water O atom form the coordination environment of the Na(+) ion. This arrangement forms Na-O-Na layers in the crystal structure, parallel to (100).