This document specifies a method, using formic acid and zinc chloride, to determine the mass percentage of viscose or certain types of cupro or modal or lyocell, after removal of non-fibrous matter, in textiles made of mixtures of viscose or certain types of the cupro or modal or lyocell fibres with flax fibres. If a cupro or modal fibre is found to be present, a preliminary test should be carried out to see whether it is soluble in the reagent. This document is not applicable to mixtures in which the flax fibre has suffered extensive chemical degradation, nor when the viscose, cupro, modal or lyocell fibre is rendered incompletely soluble by the presence of certain permanent finishes or reactive dyes that cannot be removed completely.