The X-ray structure determination of tetraphenylphosphonium trans-aquatetrabromo(carbonyl)osmate(III) trans-[(C6H5)(4)P][OsBr4(CO)(H2O)], at 293 K has revealed slightly distorted octahedral coordination of the osmium(III) atom, with H2O trans to the almost linear carbonyl group. The distances are 1.795 (10) for Os-C, 2.181 (6) for Os-Ow and 1.164 (9) Angstrom for C--O; among the Os-Br bond lengths two are shorter [2.4692 (8) and 2.4753 (10) Angstrom] and two are longer [2.5042 (8) and 2.5072 (8) Angstrom]. Centrosymmetric dimers of the complex anions are formed by intermolecular hydrogen bonding between the coordinated water molecule and the two Br ligands that form longer bonds to the central atom.