Mg2Ru2Cl10O.16H2O {dimagnesium mu-oxo-bis[pentachlororuthenate(IV)] hexadecahydrate} crystallizes in the monoclinic system (space group P2(1)/c). The structure consists of layers of [Ru2Cl10O]4- anions, [Mg(H2O)6]2+ cations and water molecules stacked along the a axis. Only the O atom bonded to Ru occupies the 2a site with -1 symmetry. All the other atoms occupy general 4e sites. The crystal structure is stabilized by O-H...O and O-H...Cl interactions.