This document specifies a method, using diluted acetic acid, to determine the mass percentage of chitosan fibres, after elimination of non-fibrous matter, in textiles made of mixtures of: chitosan fibre with certain other fibres. This method is applicable to fibre mixtures of chitosan fibre with cellulose fibres (cotton, linen, ramie, viscose, modal, lyocell), protein fibres (wool, cashmere, silk), or synthetic fibres (polyester, polyamide, acrylic).