The crystal structure of M g S 0 3 H20 , space group P 2 \ln, Z = 4, Dx = 2.415 g em -3 , a = 4.699(1), b = 12.751(3), c = 5.618(1) Ä, ß = 90.49(3)°, was determined by single crystal X-ray diffraction. M g S 0 3 H20 crystallizes in the M n S 0 3 H20 type. The structure consists of buckled trans layers ^ [M gS03 H20 ], which are built up from strongly distorted M g 0 5(H20) octahedra sharing four equatorial vertices, and of trigonal pyramidal S 0 32-ions. It is close ly related to the structures of orthorhombic M nSe0s3 D 20 and monoclinic Z n S e 0 3 H20 . The M g -0 distances range from 2.051(3) to 2.175(4) A. The S-O distances (1.543(3), 1.547(3) and 1.493(3) Ä) and the O-S-O angles (98.4(2) and 2x 106.0(2)°) correspond to those in M n S 0 3 H20 . The distortion of the M 0 5(H20) octahedra (M = Mg, Mn) and of the S 0 32-ions is smaller in M g S 0 3 H20 , but with greater deviations from m symmetry. The distances between the H-connected ^ [M S 03 H20 ] layers are greater in M g S 0 3 H 20 , indicating weaker inter-layer hydrogen bonds. The lateral arrangement of the 2 [M S 0 3 H20 ] layers is nearly the same in both sulfite monohydrates. E in le itu n g