Manganese diselenite, (\mathrm{MnSe}{2} \mathrm{O}{5}), exhibits two allotropic forms. The structure of the monoclinic (m) variety has been determined (\left(a=7.960(3), b=10.938(1), c=5.850(2) \AA \AA, \beta=118.27(2)^{\circ}, Z=4\right.), space group (=C 2 / c) ) and compared with the structure orthorhombic one, (o). Both structures present similarities: (1) zigzagging chains of octahedra (\left[\mathrm{MnO}{6}\right]), running along the ([001]) axis; (2) each octahedron shares opposite edges with the two neighbor ones; (3) (\mathrm{Mn}-\mathrm{Mn}) intrachain distances are similar, 3.4422(8) (\AA) for the monoclinic vs (3.394(2) \AA) for the orthorhombic; (4) each chain is interconnected with six other chains via (\mathrm{Se}{2} \mathrm{O}{5}^{2-})-anions. Differences appear between the (m) and (o) forms are also apparent: (1) the zigzagging angle is sharper in the former; (2) the bridge angle (\mathrm{Se}-\mathrm{O}-\mathrm{Se}) is higher in the (m) form in comparison to other diselenites including the (o) form; (3) the interchain distances range from 5.169 (\AA) to (7.965 \AA) for (m), and between (6.190 \AA) to (6.822 \AA) for the (o) form. Differences are also noticed by infrared spectra. The phase transformation (m \Leftrightarrow o) does not occur by heating, even up to decomposition. A magnetic study of both varieties shows a weak antiferromagnetic interaction between manganese ions along a chain and reveals the occurrence of a long range antiferromagnetic ordering at significantly different temperatures: (T{\mathrm{N}}=9) and (5.2 \mathrm{~K}) for (m) and (o) species, respectively. A magnetostructural correlation is proposed. 1993 Academic Press, Inc.