This study evaluated whether the reinforcement effect of bleached softwood kraft pulp fibres added to a thermomechanical pulp (TMP) -based furnish for light weight coated (LWC) paper could be improved by high consistency. (HC) refining. The reinforcement pulp was HC refined under various temperature and steam-pressure conditions. Fibre and sheet properties, such as fibre length, fibre curl, tensile strength, strain at break and tensile energy absorptioh, of both isotropic handsheets and orthotropic paper were studied. The reinforcement effect was estimated by fracture-mechanics for a larger web containing a small crack. The critical force and critical elongation were calculated based on material parameters measured on laboratory made orthotropic papers. The study revealed that HC refining did not significantly increase the final level of fibre curl of bleached softwood kraft fibres after a combination of high- and low-consistency refining. Despite this, testing of laboratory sheets indicated that the highest levels of tensile energy absorption and strain at break were achieved as a result of HC refining. The highest level being achieved with fibres refined in the pressurised refiner at high temperature. Testing of orthotropic sheets indicated that it is possible to increase the strain at break, tensile energy absorption index, critical' deformation and critical load of sheets made of a TMP furnish reinforced with HC refined kraft pulp (70% TMP and 30% kraft pulp), the disadvantage of such reinforcement is a decline in tensile index. The best combination of critical load and critical elongation was achieved with reinforcement pulp HC refined in a steampressurised system at 125°C. Both undried and dried kraft pulps were investigated: the dried pulp generally displayed similar or better performance, regardless of whether or not it was pre-treated in a HC refiner before the low consistency (LC) refining stage or not. Pore size and pore size distribution in the fibre wall were determined by studying the relaxation behavior of 2H in 2H2O-saturated fibres using NMR spectroscopy. This was done in order to analyse the effect of high consistency refining on the fibre wall ultrastructure. The result indicated that the pore size decreases in fibres subject to HC refining, higher temperatures during HC refining resulting in smaller pores. HC refining thus produces effects resembling hornification in the fibres.