Abstract: This invention relates to a three-dimensional, aromatic, sulfur-containing polymer based on lignin. The polymer is characterized in that it is amorphous, practically insoluble in water and conventional organic solvents; it contains thioether bonds and possibly sulfoxide and/or sulfone groups formed by oxidizing of thioether bonds, and possibly carbon-carbon bonds between aromatic rings and/or side chains and possibly other bonds typical to lignin, depending on reaction conditions. Its thermostability is at least 200.degree. C.; and it has been obtained from lignin which is dissolved or undissolved, chlorinated or unchlorinated, purified or containing other components of spent pulp liquor, by heating at about 160.degree.-350.degree. C.