Aluminium chlorofluoride (ACF) and high-surface aluminium fluoride (HS-AlF₃) were analyzed by a set of characterization methods to assess their acidic properties: NH₃-TPD, CO adsorption followed by DRIFTS, CD₃CN-PAS-FTIR and MAS NMR spectroscopy after ¹⁵N-pyridine adsorption. Both catalysts contain very strong and medium-strong Lewis acid sites as confirmed by CO adsorption, in which small differences arise from the morphological properties of each catalyst, with ACF being microporous and HS-AlF₃ mesoporous. Shifts of the CO vibration band of up to 77 cm⁻¹ were observed, which account for very strong Lewis acid sites. In addition, very strong Lewis acid sites could be identified by CD₃CN-PAS for both catalysts, exhibiting a shift of 95 cm⁻¹ from free nitrile, the highest ever reported for a solid Lewis acid.