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Hydrogen peroxide represents an ecologically and economically acceptable agent for bleaching of flax fibers. The influence of hydrogen peroxide treatment under different conditions, i.e. hydrogen peroxide concentrations (1%, 2% and 4% w/v) and treatment temperature (50 degrees C, 80 degrees C and boiling temperature), on the chemical composition, electrokinetic and sorption properties and whiteness index of flax fibers, has been studied. The surface properties and water uptake behavior, i.e. swelling of untreated and treated flax fibers, were monitored through zeta potential measurements using the streaming potential method.The present research has found out that hydrogen peroxide simultaneously removes hemicelluloses and lignin from flax fibers. The ratio between the removal of hemicelluloses (hydrophilic component) and lignin (hydrophobic component), as well as changes in crystallinity, pore structure and carbonyl and carboxyl groups content, have a dominant effect on the electrokinetic, i.e. zeta potential versus pH and isoelectric point and sorption properties of the treated flax fiber. An increase of approximately three-to-four-fold in the whiteness index of the treated flax fibers has been observed. The established correlations between the modification conditions and properties of flax fibers, allow the utilization of hydrogen peroxide for bleaching and simultaneous fiber modification with the possibility of tailoringflax fibers properties. Graphic abstract