This paper examined the effect of several pyridinium and imidazolium-based ionic liquids (ILs) on the protease stability in aqueous solutions. In general, the enzyme was found quite active at low concentrations of hydrophilic ILs. In aqueous environment, the enzyme was stabilized by the kosmotropic anions (such as CF3COO- and CH3COO-) and chaotropic cations (such as [BuPy](+) and [EMIM](+)), but was destabilized by chaotropic anions (such as tosylate and BF4-) and kosmotropic cations (such as [BMIM]+). (c) 2005 Elsevier Inc. All rights reserved.