The structures of two modifications, (I) and (II), of potassium dihydrogenphosphate-hydrofluoric acid (1/1), KH(2)PO(4).HF, were determined at 250 and 150 K, and at 292 and 150 K, respectively. Modifications (I) and (II) crystallize from stoichiometric aqueous solutions at 295 (1) and 308 (3) K, respectively. The H atoms were located clearly from the difference Fourier maps in each modification. The two modifications differ mainly in the arrangement of the dihydrogenphosphate anions, i.e. (I) contains looped dimeric and tetrameric units of the dihydrogenphosphate ions, whereas (II) contains two types of looped tetrameric unit. In addition, both structures contain a very short F-H.O hydrogen bond (2.38-2.40 A). The K(+) ions are coordinated by O and F atoms, with similar K.O and K.F distances in both modifications.