The title compound (systematic name: trans-hex-3-enedioic acid), C(6)H(8)O(4), (I), is located on an inversion centre. A less accurate room-temperature structure has been reported previously [Ganis & Martuscelli (1966). Ric. Sci. 36, 439], without the position of the H atom involved in hydrogen bonding. The carboxylic acid groups of (I) link molecules across inversion centres through O-H...O hydrogen-bonded pairs. The result of these interactions is the formation of chains with the graph-set description C(2)(2)(18), which run along the c axis. The significance of this study lies in the analysis of the interactions occurring via hydrogen bonds, as well as in the comparison drawn between the molecular structure of (I) and the structures of several other acid derivatives possessing two carboxylic acid groups.