Chloroaluminate ionic liquids (ILs), especially composite ionic liquids (CILs) which are ILs modified with copper(I) chloride, are highly active and selective catalysts for the alkylation of 2-butene with isobutane. The Lewis and Bronsted acidic species of these ILs were investigated by NMR spectroscopy. Pyridine is found to be a suitable indicator. Lewis acidity arises mainly from Al2Cl7- having a chemical shift at 102 ppm in the Al-27 NMR spectrum, while Bronsted acidity arises from Al2Cl6OH- (chemical shift at 97 ppm). The peak at 94 ppm in the Al-27 NMR spectrum is related to Al2Cl5O-. These new insights have improved our understanding of the structure of the active species in chloroaluminate ionic liquid alkylation catalysts. (C) 2014 Elsevier Inc. All rights reserved.