Ionic liquids (IL) offer new possibilities for solvent engineering for biocatalytic reactions. The deracemization of (+/-)-mandelic acid using a lipase-mandelate racemase two-enzyme system was used to investigate the scopes and limitations of ionic liquids as new reaction media for a dynamic resolution approach. Mandelate racemase [EC 5.1.2.2] from Pseudomonas putida ATCC 12633 was observed to be active in ionic liquids such as 1,3-dimethylimidazolium methylsulfate ([MMIM][MeSO4]) or 1-butyl-3-methylimidazolium octylsulfate, ([BMIM][OctSO(4)]) at water activities a(w) > 0.74. Mandelate racemase activity could also be obtained in a biphasic system consisting of water and 1-octyl-3-methylimidazolium hexafluorophosphate ([OMIM][PF6]) in a ratio of 1:10. (C) 2004 Elsevier B.V. All rights reserved.