Abstract Modified flax fibres may represent an interesting alternative to wood fibres derived from kraft pulping, being substitutes for asbestos and synthetic fibres. The aim is to produce flax fibres of the same quality as sulphate wood pulp by means of a simple and environmentally correct method and at a competitive price. It is of fundamental importance to be able to control the raw material quality during the retting period after the flax has been combine-harvested. The most important factor consists in the determination of the optimum stage of retting in the field; the stage that will result in the highest and best quality for industrial purposes. A processing method that differs from a traditional chemical method is developed and consists in mechanically treating the flax straw in a hammer mill followed by a process separating the fibres from the shives (the woody core of the stem). Subsequently, the shortened flax fibres are exposed to a hydrothermal treatment resulting in a highly cellolytic product. The best quality according to chemical composition, fibre percentage, degree of purity and fibre strength is obtained from moderated retted flax. Experiments involving ammonia (NH3) preservation of flax straw during wet harvest seasons look promising.