Deacidification, i.e., neutralizing acids in paper and impregnating an alkaline buffer, is the normal method of stabilizing an impermanent acidic paper. The fundamental difference between alkaline sizing and deacidification is the point at which neutralization and introduction of an alkaline buffer occur. Alkaline sizing occurs during papermaking, deacidification occurs after the product is delivered to the customer. All of the paper is protected during alkaline sizing; especially selected items are protected by deacidification. The exceptional permanence of fibers exposed to very acidic conditions in wood is reported. Alum is identified as changing in the mid-19th century from being the panacea for paper preservation to being the primary cause of paper deterioration. Other important sources of acidity in paper and the effect of acids on cellulose are discussed. The proposed and operating mass deacidification systems: 1) Library of Congress Diethyl zinc; 2) Preservation Technologies Bookkeepers; 3) Book Preservation Associates Ethanolamine; 4) Interleaf V.P.D. Process; and 5) Wei T'o Nonaqueous Book Deacidification System are described; characteristics, advantages, and disadvantages noted and sources for further information listed. -- AATA