KHCOO II is orthorhombic at ambient conditions and it is shown that traces of moisture affect the polymorphism of these very hygroscopic crystals. Dry KHCOO II transforms into phase I at 417 K (144 °C), and this phase can be supercooled to room temperature, remaining metastable for several days before transforming back to the orthorhombic phase II. The Raman spectra of phases I and II, as well as of supercooled phase I, are reported in the present study. The absence of some prominent translational modes in the Raman spectra of KHCOO II, compared to NaHCOO II, can be explained on the basis of a group-theoretical treatment. From the temperature dependence of the linewidths of various Raman-active librational and internal modes, activation energies are obtained for intramolecular motions of the formate ions. Fermi resonance occurs between the overtone of the bending mode 2ν5 and the C—H stretching mode ν1 in KHCOO II and the coupling constant W increases with temperature. The Raman and X-ray data show that KHCOO I is structurally different from NaCHOO I, but it is not possible to assign a definite structure to this phase on the basis of a Raman spectrum alone. Key words: Potassium formate, phase transition, Raman spectra.