The crystal structure of thorium nitrate pentahydrate, Th(NO3)4.5H20, has been determined by neutron diffraction. It is orthorhombic, space group Fdd2, with a = 11.191 + 0.007/~,, b = 22-889 + 0.015/~ and c= 10.579+0.007/~,. A total of 1642 independent reflections with 20 < 122 ° 0~= 1.065/~) were used. The structure was solved from the three-dimensional Patterson function and refined by the full-matrix least-squares method to R = 4.3 % with the use of a statistical weighting scheme.Eleven oxygen atoms are bound to the thorium atom, eight being from four bidentate nitrate groups and the other three from water molecules. The Th-O (water) distances, 2-438 and 2.473/~,, are shorter than the Th-O (nitrate) distances, 2-528 to 2.618/1. The nitrate groups are planar and the N-O distances are found to be significantly different within the group. The structure is tied together by a simple hydrogen-bond scheme in which the hydrogen bonds are either 'strong' water-water hydrogen bonds (2-70/~) or 'weak' water-nitrate oxygen hydrogen bonds (2.90-2.95 •).