Temperature-programmed desorption of ammonia has been employed for the characterization of halogen exchange catalysts. Nonactivated and activated catalysts are compared in regard to their behavior in adsorption and desorption. Activation of gamma-alumina and AlF2(OH) increases both the catalytic activity for halogen exchange and the amount and strength of ammonia adsorption. In constrast, beta-AlF3 possesses catalytic activity already without activation. This substance reveals no significant alteration of ammonia adsorption after activation. A connection was observed between the acidity of the catalysts and their reactivity. The nature of the acidic sites was determined by Fourier transform infrared photoacoustic spectroscopy of pyridine chemisorbed on the solid surfaces. The results indicate that Lewis acid sites of a certain strength are responsible for the catalytic activity.