The object of this project was to evaluate the effect of high sulphidity on the performance of the kraft cooking process. Softwood (SW) mix (50/50 of spruce, Picea abies, and pine, Pinus sylvestris) and a hardwood (HW: birch, Betula pendula) have been explored as wood raw material. The reference kraft cook was carried out at 35% sulphidity. The laboratory kraft cooking trials showed that the rate of delignification in kraft cooking increases with increasing sulphidity for both these raw materials. An increase in sulphidity from 35% to 80% increases the apparent delignification rate by a factor of almost two. The faster delignification rate could be exploited as an increase in production and/or as a reduction in EA charge (2%-units for both raw materials) and/or as a decrease in cooking temperature (5°C for SW) at the same production. The yield of softwood kraft pulp at a given kappa number was not affected by the increase in sulphidity. For hardwood, the combination of high sulphidity and reduced effective alkali charge gave an increase in pulp yield of one %-unit. The pulp viscosity at a given kappa number was increased. The high sulphidity in SW kraft cooking (at a given EAcharge) led to a slightly better bleachability in an OD(E+P)DD bleaching sequence. The fully bleached SW kraft pulp at 80% sulphidity had a slightly higher tear index and higher fibre strength than the corresponding reference kraft pulp at 35% sulphidity. The other strength properties were virtually unchanged. The fully bleached HW kraft pulp at 80% sulphidity showed better optical properties, light scattering and opacity, which are, in fact, the critical properties for fully bleached hardwood pulps.