A difference in the absolute brightening response was observed between predominantly juvenile and mature kraft fibers of a Northern black spruce following oxygen and peroxide bleaching. Although the initial brightness from the digester for the two wood fiber types is approximately the same (36.8 vs 36.5 for juvenile and mature, respectively), it is interesting to note that the juvenile wood has a lower brightening response than the mature wood. A potential source for the brightness difference may be the increased amount of compression wood found in the juvenile wood. Another potential source for the brightness difference may be the chemical characteristics of the lignins. The juvenile and mature pulp lignins display substantial differences in methoxyl content; the mature lignin sample possessed a greater number of methoxyl units. The methoxyl group would be expected to increase the activation energy of the aromatic rings to addition reactions in both peroxide and oxygen bleaching reactions. Further lignin differences included a higher amount of aliphatic hydroxyl groups in the juvenile lignin, while mature lignin contained a greater amount of carboxylic acid groups. Metal data from inductively coupled plasma measurements indicated that the juvenile wood is richer in manganese and iron, perhaps related to lignin functional differences that may account for inefficient consumption of peroxide.