Abstract: Cellulosic material present in admixture with or as impurities in wool or other non-carbonizable fibres is carbonized by treating the fibre substrate, which may be in any form from loose fibre to fabric, with a solution of an anhydrous acid in an inert organic solvent. The treated fibrous substrate is then dried and can be beaten, rinsed and further dried if necessary. The preferred solvents are aliphatic hydrocarbons or chlorinated aliphatic hydrocarbons and the acid may be an inorganic acid such as hydrochloric acid or an organic acid such as trichloracetic acid. The method can be applied to unscoured or undesized wool, in which case the wool is cleaned at the same time as cellulosic materials are being carbonized. The solvent may advantageously contain up to 100 g/l trioxan. The method can be applied by impregnating the fibrous substrate with the organic solvent and subsequently exposing the impregnated substrate to the acid, which may be gaseous hydrogen chloride.