A naturally decayed birch stem was investigated in order to find thus also indirectly for the photo-chemical yellowing of mechanical pulps. The results are compared to those for fresh birch. The density of the decayed birch was one quarter that of the fresh birch and its luminance value was higher. The chemical composition of the decayed birch differed somewhat from that of the fresh birch Solid state C-13 NMR spectra indicated that the lignin of the decayed birch had a lower acetyl content and a slightly higher content of free phenolic units. Milled wood and enzymatically isolated lignins from the fresh and decayed birchwoods were investigated. The results indicated that the decayed birch lignin contained fewer syringyl units, more alpha-carbonyl groups probably in the carboxylic acid groups. The light stability of the decayed wood can be partly explained by the occurrence of carboxylic acid groups in the alpha-position of the lignin side chain and by assuming that the groups that are easily yellowed photochemically dissolve during the decay, leaving behind a stable and perhaps more rigid macromolecule.