A process and apparatus for transforming and/or extracting lignin or its decomposition products from lignin-cellulosic materials. For this purpose, a redox potential in the 200-500 mV range is set by adding oxidizing and/or reducing agents and/or salts and/or phenolic compounds to an acid aqueous solution of the lignin-containing raw materials. The lignin-decomposing reaction is initiated with bleaching by adding enzymes, micro-organisms, animal or vegetable cells. The reaction is maintained for several hours at a redox potential value oscillating around a constant value, at a constant temperature and with constant agitation.
