The structure of the very strong solid Lewis acid aluminum chlorofluoride (ACF, AlClxF3-x, x = 0.05-0.3) was studied by IR, ESR, Cl K XANES, F-19 MAS NMR, and Al-27 SATRAS NMR spectroscopic methods and compared with amorphous aluminum fluoride conventionally prepared by dehydration of alpha-AlF3.3H(2)O. The thermal behavior of both compounds was investigated by DTA and XRD. In comparison to ACF, amorphous AIF(3) prepared in a conventional way is not catalytically active for the isomerization reaction of 1,2-dibromohexafluoropropane, which requires a very strong Lewis acid. Both compounds are mainly built up of corner-sharing AIF(6) octahedra forming a random network. The degree of disorder in ACF is higher than in amorphous AIF(3). Terminal fluorine atoms were detected in ACF by F-19 NMR. The chlorine in ACF does not exist as a separate, crystalline AlCl3 phase. Additionally, chlorine-containing radicals, remaining from the synthesis, are trapped in cavities of ACF. These radicals are stable at room temperature but do not take part in the catalytic reaction.