The natural aging of cellulosic materials (high purity paper, cotton, raw and bleached flax, viscose) was simulated by thermal oxidation in air at constant temperature (123 -192 8C). Also the synergetic effect of light was investigated at 50 -85 8C. The kinetics of polymer degradation was followed through intrinsic viscosity measurements and was correlated with the degree of crystallinity, determined by X-ray diffraction, and with the surface oxidation of the investigated samples, evaluated through XPS analysis. At least two constant rate stages were observed in kinetic runs. The acceleration factors and time necessary for simulating a 20-year thermal aging of the investigated cellulosic materials were obtained from the calculated activation parameters by extrapolation of the process rate coefficients to room temperature. Surface oxidation parallel to aging led to the formation of low-molecular-weight, easily removable products in pure cellulose materials, while in lignin-containing materials mainly lignin was involved. ZUSAMMENFASSUNG: Die natu Β¨rliche Alterung von Zellstoffen (hochreines Papier, Baumwolle, ungebleichter und gebleichter Flachs, Viskose) wurde mittels thermischer Oxidation bei konstanter Temperatur (123 -192 8C) simuliert. Die synergistische Lichteinwirkung bei 50 -85 8C wurde ebenfalls gepru Β¨ft. Die Kinetik des mit Hilfe von Viskosita Β¨tsmessungen verfolgten Abbaus der Polymeren wurde mit dem durch Ro Β¨ntgendiagramme ermittelten Kristallinita Β¨tsgrad sowie mit der durch XPS