The study of the enzymatic oxidation of lignin samples by hydrogen peroxide in the presence of horseradish peroxidase in the water - DMSO medium was carried out. Study was performed using the spectrophotometeric method and size-exclusion liquid chromatography. The possibility of the enzymatic oxidation of lignin in presence of 30 % DMSO was shown. The effect of enzymatic oxidation on molecular-mass distribution of lignin was investigated. The influence of lignin concentration on initial reaction rate and maximum absoibance was discussed.