Four mass deacidification processes for book preservation, namely the Library of Congress diethyl zinc process, Wei T'o nonaqueous, Kopper's Book Keeper process, and Langwell interleaf vapour phase process are critically evaluated, based on their chemical characteristics and effectiveness on deacidification. It is concluded that none of the processes totally satisfies all the requirements and that independent testing should be carried out by a group none of whom is associated with promtion of any of the potentially useful processes.