In the crystal structures of two cyclic trihydroxamic acid derivatives containing the same substructure unit, viz. 1,3,5-trihydroxy-1,3,5-triazinane-2,4,6-trione dihydrate, C3H3N3O6.2H2O, (I), and 1,3,5-benzyloxy-1,3,5-triazinane-2,4,6-trione, C24H21N3O6, (II), there is no significant difference in the geometric parameters. In (I), there are 11 hydrogen bonds of the O-H...O type interconnecting the molecules in a three-dimensional network, while in (II) there are only two weak C-H...O hydrogen bonds. The results of IR spectroscopic analysis are in good agreement with the crystallographic study.