Abstract: Method of treating a biomass containing a fibrous structure and polysaccharides. According to the method the biomass is subjected to extractive treatment at which it is heated in aqueous phase at an absolute pressure of over 0.6 MPa and a temperature of at least 160° C. for separating polysaccharides from the mass. According to the invention, the pH of the biomass which is subjected to the extractive treatment is not actively lowered during the extraction and after the treatment the pressure is controllably reduced so as not to decompose the fibrous structure of the biomass. The polysaccharides separated from the mass are recovered as a first fraction, the fibrous structure of the biomass as a second fraction and at least one of the fractions is subjected to further processing. By means of the invention it is possible to isolate from the starting materials valuable compounds and groups of compounds by a method suitable for use on an industrial scale.