The kinetics and mechanism involved in the kappa number determination have been studied using a spectrophotometric method to monitor the oxidation of pine kraft pulp as well as of model compounds for lignin and carbohydrates. It was found that, during the kappa number determination, the permanganate oxidation proceeds stepwise, forming manganese dioxide which is then reduced to manganous ions. Lignin is oxidized by the addition of permanganate to all the double bonds in the aromatic rings followed by a rupture of the rings. The predominant part of the oxidation of lignin is completed within one minute whereas the oxidation of e.g. glucose is slow but proceeds at an accelerating rate during the kappa number determination time (10 min). The spectrophotometric determination of per manganate remaining after the reaction has advantages over the traditional iodometric method.