The Raman spectra corresponding to the internal vibrations of the ammonium ion in NH4MnF3 and NH4ZnF3 single crystals have been measured from 10 to 300 K. The spectra clearly show the existence of a structural phase transition from cubic to tetragonal symmetry. The splitting of the internal modes is compatible with effective symmetries of the ammonium ion of D2d and Cs in the cubic and tetragonal phases, respectively. The temperature dependence of the symmetric stretching mode in the cubic phase is linear with a slope of 0.231 cm-1 K-1 for NH4MnF3. This has the opposite sign to that due only to the crystal expansion as obtained by applying hydrostatic pressure. A sharp band appearing only in the spectrum of NH4MnF3 just below the crystallographic phase transition originates in a strong symmetry-induced v2+v4:v1 Fermi resonance. © 1988 IOP Publishing Ltd.