The Bronsted acidities of sulfortic acid-functionalized ionic liquids (SFILs) with pyridinium cations were investigated. The Hammett function values of SFILs are between -1.5 and -3.6 at 110 degrees C. The acidities of SFILs depend on the anions, and have the order as follows: [PSPy][BF4]>[PSPy][HSO4]>[PSPy]LpTSA]>[PSPy][H2PO4] ([PSPy]=N-propane-sulfonic acid pyridinium). The acidity order of SFILs is consistent with their activity order observed in some acid-catalyzed reactions. The minimum-energy geometries of SFILs manifest that anions have strong interactions with the sulfonic acid proton. It is considered that in addition to the alkyl sulfortic acid group, the anion is likely to serve as available acid sites. Hence the acidities and catalytic activities of SFILs depend on the kinds of anions. (c) 2006 Elsevier B.V. All rights reserved.