The triclinic lattice of barium perrhenate monohydrate, Ba(ReO 4 ) 2 .H 2 O, is metrically pseudomonoclinic. The structural motif resembles the P2/c space group pattern with one Ba(ReO 4 ) 2 .H 2 O formula in the pseudoasymmetric unit. Ba atoms are tenfold coordinated by comers of different ReO 4 tetrahedra and two water O atoms. The BaO 10 polyhedra are associated in pairs through a triangular face which includes the twofold pseudo-axis. The large Ba cations and ReO 4 anions are ordered in infinite columns of face-sharing Ba 2 Re 4 octahedra running along [100] in a quadratic mesh. The intercolumnar space is occupied by the water molecules.