Due to excellent mechanical properties, high chemical stability and processability polypropylene fibres are mostly used in different technical fields. However, because of low surface energy, lack of reactive sites and sensitivity to photo or thermal oxidation the polymer properties are insufficient for some applications. Therefore several techniques for fibres modification are reported, e. g., plasma treatment, chemical modification etc. To alter the chemical and physical properties hydrophobic polypropylene monofilament fibres were surface modified and the sulphonation with concentrated sulpuric acid was used. The sulfonation was performed at different conditions, e. g., different temperatures and varying period of time and a detailed study of treatment conditions influence on the fibre properties was carried out. The sulphur content on fibres was determined by mass spectroscopy. The sulfonic acid cation exchange polypropylene fibres were characterised in terms of elektrokinetic and mechanical properties. (C) 2003 Kluwer Academic Publishers.