Wood pulp fibres are promising reinforcements for biocomposites due to their renewable resource origin and mechanical properties. An oriented and dense fibre reinforcement structure is beneficial for biocomposite properties. We present a method of modifying fibres (e.g. to increase strain to failure) in pre-formed oriented high-density paper structures intended for biocomposites or as hot-pressed fibre materials. Mildly delignified, well-preserved holocellulose fibres from softwood are used. Cold alkali treatment (hemicellulose removal) and mercerisation (conversion to cellulose II) were carried out successfully on oriented fibre sheets. Controlled anisotropy and sheet density are achieved from untreated and straight fibres in the sheet formation step. High mechanical properties and increased ductility of mercerised sheets were observed, which may be valuable for hot-pressed fibre materials (E ≈ 7.1 GPa, strength of 108 MPa and strain to failure of 5.3%) and biocomposites. In contrast, modified wood pulp fibres were difficult to orient, resulting in higher sheet porosity and weak interfibre bonding. Graphical abstract: [Figure not available: see fulltext.]. © 2022, The Author(s).