The activity and stability of peroxidase in aqueous solutions of two ionic liquids (ILs) have been studied. The ILs selected were 1-ethyl-3-methylimidazolium 2-(2-methoxyethoxy) ethylsulfate, [emim][MDEGSO 4 ], and 1-ethyl-3-methylimidazolium ethylsulfate, [emim][EtSO 4 ]. Experiments were performed at room temperature using concentrations of ILs between 5 and 50% (v/v) and pH values in the range from 5 to 9. The initial activity of the enzyme in these ILs at optimized conditions (pH 7 and 5-10% IL) was similar or higher than that achieved with buffer solution. Kinetic studies showed that maximum reaction velocity (V max) decreased with increasing concentration of IL. The effect of [emim][EtSO 4 ] concentration on the decrease of V max was higher than that of [emim][MDEGSO 4 ]. It was found that [emim][EtSO 4 ] was a more potent inhibitor on peroxidase activity. The peroxidase studied was active in the ILs investigated, and the enzyme exhibited a higher stability in ILs at the optimized conditions. Kinetic studies showed that inhibition was a noncompetitive type in both ILs.