Kraft lignins have been isolated from the black liquors obtained a) after normal kraft cooking of pine wood to different yield levels and b) after a flow-through cook. After purification, the two series of lignins have been subjected to oxidative degradation in a four step reaction sequence leading to the formation of substituted aromatic carboxylic acid Methyl esters. The yields and distribution of the different esters obtained from the flow-through cook indicate that in kraft cooking the lignin which goes into solution undergoes structural changes which coincide with the transition points between the initial, the bulk and the final delignification phases. From the other series of cooks, changes in the frequency of occurrence of the individual esters were also observed as the cook proceeded. These changes could not, however, be attributed to the different phases of delignification but took place rather continuously during the cook. The observed results are discussed with reference to known features of delignification chemistry in kraft cooking.