The time-dependent behavior of material that affects the kappa number of birch kraft pulp was experimentally tested in an industrial, two-stage, oxygen-alkali delignification process. The pulps were leached, and the leached material was divided into four different fractions: the wash loss fraction and the easily leachable, slowly leachable, and stagnant fractions. These fractions were further characterized according to their chemical natures, representing residual lignin, extractives, hexenuronic acids, and other chemical structures contributing to the kappa number of the pulps. The apparent and actual delignification responses in the two reactors and the effects of the leaching operation were thoroughly evaluated based on the behaviors to these different pulp components.