The accelerated aging test conducted by the Library of Congress on enclosing books and paper in plastic film is discussed; heat-aging in glass tubes and in ovens is also considered. Deterioration correlates with the exposure of cellulose to humidity; moisture stability appears to be the most important factor. Polymer films will permit equalization of humidity over a period of time, but short-term fluctuations of the outside environment will not affect the enclosure conditions so radically. Autocatalysis is a significant factor in the higher aging rate for sealed materials under accelerated aging conditions; however, there is no evidence that the same phenomenon occurs in natural aging. Use of buffered paper in an accelerated aging test is recommended to determine whether current theory of autocatalysis holds.