The new compound Hg 2FeF 5(OH) 2 · H 2O was prepared by evaporation of an aqueous 40% HF solution containing HgO and FeF 3 in the stoichiometric ratio. The material is orthorhombic, space group Cmmm, with a = 7.505(1) Å, b = 11.823(3) Å, c = 3.941(2)Å, and Z = 2. The crystal structure was determined from single crystal intensity data obtained by means of an automated four-circle diffractometer and refined to the conventional values R = 0.0621 and Rw = 0.0566 for 451 observed reflections. The structure is characterized by infinite straight chains of FeF 6 octahedra sharing trans F atoms in the direction . These chains are linked by rutile-type chains of HgF 4(OH) 2 octahedra also running along . Water molecules are statistically distributed on half of the 4 i positions; they are off-centered in the channels parallel to  allowing OH ··· F bonding. The structure is compared to that of HgFeF 5 · 2H 2O and to that of the hexagonal tungsten bronze.