AbstractChemical modifications of cellulose fibres (kraft pulp) in NaOH/H2O and NaCl/H2O systems were investigated. Handsheets were prepared that contained 25% of the modified fibres. Changes in the modified samples were examined by measuring their mechanical and optical properties and comparing them to those made with unmodified fibres. The observed differences were explained and supported by structural analyses, by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). It was found that the NaOH/H2O pretreatment led to a significant deterioration of optical and strength properties of the handsheets. These modifications affected both the inner part of the crystalline cellulose (change from cellulose I to cellulose II) and the morphology of the fibers. Conversely, these properties slightly improved after propargylation, due to the propargyl functional groups. For the NaCl/H2O system, a significant enhancement of the mechanical properties of the handsheets was noted, such as an increase of up to 108% of the tear index. The propargylation further increased the tear index (by 157%). These enhancements were not accompanied by significant changes at both the micrometric and nanometric scales, except for the increase of the crystallinity index after propargylation.