Oxidative enzymes, laccase C from Trametes sp. and horseradish and soybean peroxidases, catalyzed oxidation reactions in systems with ionic liquids whose content varied from several volume percent to almost total non-aqueous ionic liquids. Similar to the effects produced by standard organic solvents used in non-aqueous enzymology, catalytic activity of the enzymes was decreased by adding a water-miscible ionic liquid, 4-methyl-N-butylpyridinium tetrafluoroborate, or by suspending the enzyme in a water-immiscible ionic liquid, 1-butyl-3-methylimdizaolium hexafluorophosphate. For the oxidation of anthracene, catalyzed by laccase C and assisted by a number of mediators, addition of 4-methyl-N-butylpyridinium tetrafluoroborate, instead of tert-butanol, increased the yield of the oxidation product several-fold.