During kraft cooking a significant part of the wood material, especially lignin and hemicelluloses, is degraded and dissolved in the cooking liquor, rendering a broad range of degradation products with different molecular mass and functional groups. The main part of this thesis has been devoted to clarify the role of these dissolved wood components (DWC) during kraft cooking. The investigations have covered their influence on e.g. the delignification rate, pulp yield, point of defibration, unbleached pulp c olour and pulp bleachability, i.e. the amount of bleaching chemicals consumed per kappa number or lignin unit to reach a certain brightness. Both softwood (Picea Abies) and hardwood (Eucalyptus urograndis) have been studied. During kraft cooking, many reactions occur simultaneously. Therefore emphasis has been put on separating the effects of hydroxide ions, hydrogen sulphide ions, sodium ions and DWC. This has been enabled by the use of a so called constant-composition-cooking technique, which enables the use of almost constant concentrations of the cooking chemicals during the cook and also results in a very low concentration of DWC in the cooking liquor. The presence of DWC has been controlled by the addition of industrial black liquor. To further scrutinise the role of DWC, the effect of different molecular mass fractions were studied and representative model substances were used to clarify the origin of the observed effects. A kinetic study showed that the delignification rate was significantly affected by the presence of DWC in the cooking liquor and resulted in a rate increase in the part of the cook where the bulk phase dominates and a decreased delignification rate when the residual phase dominates. The increase in delignification rate was dependent on the concentration of DWC and was observed in softwood as well as hardwood kraft cooks. The rate increasing effect was investigated further by the use of ultra- and nanofiltra tion. This way the DWC was divided into fractions with different molecular mass distributions. The results showed that the increase in delignification rate related more strongly to the content of free phenolic groups in the DWC than on the total amount of DWC. By cooking in the presence of representative model substances the effect was further clarified. Aromatic structures with free phenolic groups gave a rate increasing effect while no visible effect could be seen from other structures. This supports the finding that the delignification rate relates to the amount of free phenols in the cooking liquor and shows that the phenolic functionality take active part in the delignification reactions. Free phenolic groups in the degraded lignin may explain a large part of the rate increasing effect seen from the presence of DWC. Further, the presence of DWC increased the point of defibration in a eucalyptus kraft cook and made it possible to terminate the cook at a higher kappa number with the same amount of reject. By terminating the cook at a higher kappa number it was possible to noticeably increase the fully bleached pulp yield. The content of hexenuronic acids (HexA) in the eucalyptus pulp depends on the H-factor and increases with delignification, providing that the bulk phase still dominates. Therefore, by increasing the rat e of delignification and terminating the cook at a higher kappa number it was possible to significantly decrease the amount of HexA in the pulp. The presence of DWC causes a darkening of the unbleached pulp. Bleachability in a D(EOP)DD sequence was negatively affected by the presence of DWC during pulping of softwood, while no effect was seen on the bleachability of hardwood / QC 20100920