Potassium and ammonium tetroxalates (MHOx,H2Ox,2H2O, where M = K,NH4 and H2Ox = oxalic acid) are isomorphous, and the structure of the former has been accurately determined by Haas, using X-ray diffraction. The structure of the latter has now been determined by neutron-diffraction analysis, based on 1731 independent reflexions and refined by anisotropic least-squares to R= 8·85%. In general, and where comparable, the results confirm those of Haas. The positions of the hydrogen-atoms attached to oxygen have been fixed with estimated standard deviations of ±0·007 Å, and for those attached to N, ±0·010 Å. The structure involves seven distinct hydrogen bonds between O atoms: they range in length (O O) from 2·899 to 2·472 Å, and the corresponding O–H distances tend to increase inversely from 0·945 to 1·102 Å. The four H atoms of the ammonium ion form four, nearly linear, N–H O bonds with N O lengths = 2·927, 2·938, 2·950, 2·974 (each ±0·004)Å and N–H = 1·004, 1·022, 1·015, 0·995 (each ±0·010)Å. There is a fifth N O contact of 2·973 Å, without any intervening proton. The oxalate residues have their C and O atoms coplanar, and C–C = 1·544, 1·549 (each ±0·005), and 1·549 (±0·003)Å. In the three un-ionised carboxyl groups CO = 1·206, 1·211, 1·212 (each ±0·004)Å and C–O(H)= 1·296, 1·288, 1·291 (each ±0·004)Å. The remaining ionised carboxyl group has C[graphic omitted]O = 1·230, 1·247 (each ±0·004)Å.