Singlet oxygen mediated degradation was carried out on lignin from steam-exploded straw, organosolv lignin and alkali lignin. The lignins were characterized by elemental analysis, UV, FTIR, 1H and 13C NMR, gel permeation chromatography, and thioacidolysis. The kinetics of singlet oxygen degradation were obtained by following the gel permeation chromatograms in the presence of an external standard. This procedure cannot be used for alkali lignin. In the case of steam-exploded and organosolv lignins, the reactions follow zero-order kinetics for the first 4-8 hours; thereafter, the reactions slow down and follow slower zero-order kinetics This behaviour can be explained assuming that easily degradable structures in the outer sphere of lignins are degraded in the first period, while the core lignin is degraded in the second one. The degradation in organosolv lignin is faster than in steam-exploded lignin. Molecular weight distribution and the absence ofguaiacyl units in steam-exploded lignin can give rise to this behaviour. Similar behaviour was obtained in the singlet oxygen degradation of the pulp of steam-exploded straw.