The crystal structure of ammonium oxotrioxalatoniobate (NH4)3NbO(C204)3.H20 has been determined by X-ray diffraction. Crystals are triclinic with a=9.20+0.01, b=13-57+0.02, c=7.68+0.01 A; e=93-0+0-2, fl= 123"1 +0.2, 7 = 107"9+0"2°; Z=2; space group PT. The coordination polyhedron of niobium is a pentagonal bipyramid. Steric interactions between the ligands produce a displacement of the niobium out of the equatorial plane. The Nb-O (terminal) distance is 1.71 A and the Nb-O (oxalate) mean distance is 2.13 A..