Ammonium hydrogen tartronate at 240 and 20 K.

The low-temperature structure determination of the title compound, alternatively called ammonium hydrogen hydroxypropanedioate, NH(4)(+) x C(3)H(3)O(5)(-), has revealed that the H atom involved in a very short asymmetric O--H...O hydrogen bond [O...O = 2.448 (2) A at 240 K and 2.4393 (10) A at 20 K] is disordered.