Penicillium expansum lipase (PEL) was used to catalyze biodiesel production from corn oil in [BMIm][PF6](1) (an ionic liquid, IL) and tert-butanol. Both systems were optimized in terms of MeOH/oil molar ratio, reaction temperature, enzyme loading, solvent volume, and water content. The high conversion obtained in the IL (86%) as compared to that in tert-butanol (52%) demonstrates that the IL is a superior solvent for PEL-catalyzed biodiesel production. Poor yields were obtained in a series of hydrophilic ILs. Addition of salt hydrates affected biodiesel production predominantly through the specific ion (Hofmeister) effect. The impact of methanol on both activity and stability of PEL in the IL and in hexane was investigated, in comparison to the results obtained by two commonly used lipases. Novozym 435 and Lipozyme TLIM. The results substantiate that while different lipases show different resistance to methanol in different reaction systems, PEL is tolerant to methanol in both systems. (C) 2010 Elsevier Ltd. All rights reserved.