The Raman and infrared spectra of the fluorosulfites of K+, Rb+, Cs+, NH4+, and Me4N+ have been examined. Previous assignment of the fundamental;vibrations is revised, and an ab initio study of the SO2F- anion is presented For the pyramidal anion of symmetry C-s, distances of r(S-O) = 1.458 Angstrom and r(S-F) 1.698 Angstrom have been calculated. The heat of formation of Me4NSO2F (-14.0 kcal/mol) was derived from the dissociation pressure of the salt. In addition, the lattice energy of Me4NF (159.2 kcal/mol) was calculated from a thermochemical cycle. The thermochemical data are discussed in terms of reactivity of the fluorides and stability of the fluorosulfites.