Pyridine can act as a basic indicator for both Bronsted and Lewis acids forming different types of bonds. Infrared spectroscopy, in combination with Al-27 NMR spectroscopy, was employed to study the interaction mechanism of pyridine and chloroaluminate ionic liquids (ILs). The acid type, acid amount, acidity, and even activity of chloroaluminate ILs could be determined. An in situ infrared complexometric titration was developed as a quantitative characterization method for determining the different acidities and catalyst activity. The method is based on the principle that Lewis acidic chloroaluminate [Al2Cl7](-) anion reacts with pyridine to form the [AlCl4](-) anion and pyridine-AlCl3 ([PyAlCl3]) complexes, while the Bronsted acid forms a pyridinium salt. Both product types showed characteristic shifts of the pyridine peaks in the infrared spectrum.