The FT-IR absorption, FT-Raman scattering, X-ray powder diffraction (XPD), and thermogravimetry were used to explore the non-stoichiometry in LaOF. The TGA-DTA analyses between 30 and 1500°C showed that the LaO 1 - x F 1 + 2 x phases yielded the stoichiometric LaOF as an intermediate product. The temperature of formation of the LaOF and La 2 O 3 increased with increasing excess of fluoride.The room temperature XPD data in 6.5 2θ 121° range were analyzed by the Rietveld profile refinement method and subsequently by the bond valence calculations. All LaO 1 - x F 1 + 2 x phases possess the tetragonal PbFCl-type structure (space group : P4/nmm; Z = 2) while the stoichiometric LaOF has the hexagonal SmSI-type structure (space group : R3m; Z = 6). The unit cell parameters a and c as well as the global instability index (GII) values increased with increasing excess fluoride. The GII value for each LaO 1 - x F 1 + 2 x exceeded the limit (ca 0.2) for a possible breakdown of the tetragonal structure indicating the inherent instability of these phases.The room temperature FTIR and FT-Raman spectra between 100 and 500 cm - 1 and 50 and 1000 cm - 1 , respectively, were investigated using the factor group analysis to get more information of the local structure in LaOF. All four IR modes (2A 2 u + 2E u ) for the both forms and five of the six Raman modes (3A 1 g +3E g and A 1 g +2B 1 g +3E g ) for the hexagonal and the tetragonal form, respectively, were observed. The Raman spectra of the tetragonal LaOCl and all LaO 1 - x F 1 + 2 x were found remarkably similar.