F-19 solid-state NMR spectra of the alkali metal fluorosulfites KSO2F, RbSO2F, alpha -CsSO2F, and beta -CsSO2F show that the fluorosulfite anion is subjected to dynamical disorder at room temperature, This disorder can be modeled by 120 degrees rotational jumps with respect to the C-3-Pseudoaxis of the anion, The exchange frequency of this disorder decreases with decreasing temperature. The calculated jump frequencies at room temperature are 5 x 10(5) Hz for KSO2F, 2 x 10(5) Hz for RbSO2F, 1 x 10(7) Hz for alpha -CsSO2F, and 5 x 10(5) Hz for beta -CsSO2F, respectively. The crystal structures of alpha-and beta -CsSO2F were determined for the First time (alpha: Pnma; Z = 4; a = 790.98(8), b = 666.07(7), c = 798.93(9) pm; T = 293 K. beta: R3m; Z = 3 a = 659.22(6), c = 800.50(4) pm; T = 293 K), X-ray single-crystal data of KSO2F, RbSO2F, and alpha -CsSO2F taken at various temperatures were refined using the disorder model developed from the NMR results. It is shown that it is impossible to obtain reliable structural parameters of the SO2F- ion by refinement of X-ray data as long as rotational disorder persists, The preference of an ordered structure increases with decreasing temperature, The degree of disorder remaining in alpha -CsSO2F at 93 K is small, The structural parameters of the fluorosulfite anion as obtained by a routine refinement of this phase (S-F, 169.0(2) pm; S-O, 147.8(1) pm) are the best lower (S-F) and upper (S-O) limits of the "true" bond lengths determined experimentally. They comply with the values obtained from quantum chemical calculations.