From solutions of MnF 3 in hydrofluoric acid, MnF 3·3H 2O is formed in two polymorphic forms whose crystal structures were determined by X-ray diffraction on single crystals. Form I: space group P2 1/ c, Z4; lattice constants a8.413(1), b9.618(1), c6.370(1) Å, β97.71(1)∘; R/wRO.0417/O.0295 for 710 reflections. Form II: space group P2 1/ a, Z4; lattice constants a8.481(7), b9.473(6), c6.473(4) Å, β=100.51(6)∘; R/wR0.0303/0.0290 for 650 reflections. In both polymorphs the structures consist of centrosymmetrical complex [Mn(H 2O) 4F 2] + cations and [Mn(H 2O) 2F 4] - anions. The cations exhibit very short MnF distances ( I, 1.795 Å; II, 1.791 Å) in octahedra weakly elongated by the Jahn-Teller effect (MnO: I, 2.074/2.022 Å; II, 2.103/1.997 Å). The anions are strongly elongated along the H 2OMnOH 2 axis (MnO: I, 2.19 Å; II, 2.15 Å. MnF: I, 1.837/1.842 Å; II, 1.837/1.856 Å). Both forms of MnF 3·3H 2O have approximately the same unit cell and very similar asymmetric units but they differ in the packing symmetry documented in the different space groups and, hence, in the networks of strong OH···F hydrogen bonds (O···F: 2.62 to 2.76 Å). In I, pure cation and pure anion layers alternate along [100] direction; in II, the corresponding layers have ordered mixed cation/anion composition.