Durability of various bookbinding leathers has been evaluated after 34 years of natural exposure. Five vegetable and five chrome-tanned calfskins of specific dimensions were used to cover 60 volumes of Chemical Abstracts. The latter were bound at the Government Printing Office (U.S.A.) in the process of which hand sewing, marbling and lacing on of the covers took place. Consequently they were pasted on with a paste of the composition: flour (soft winter wheat) 19.60%; water 79.94%; phenol solution (15%) 0.11%; ammonium alum 0.35%. Afterwards a gum arabic jelly was rubbed into the backs, sides and corners. A glair consisting of pure egg albumin, 12.50%, and water to volume, was applied to the backs only, prior to gold lettering. Four dressings were applied to each leather, as follows: (1) 40% anhydrous lanolin, 60% neatsfoot oil; (2) petrolatum USP; (3) 3% Castille Soap, 12% beef tallow, 25% neatsfoot oil, 60% distilled water; (4) 30% anhydrous lanolin; 5% Japan Wax, 12% castor oil, 3% sodium stearate, 50% distilled water. It was found that chrome-tanned leathers resisted the deteriorating effects of the atmosphere better than the vegetable leathers; the latter being satisfactorily resistant only when pyrogallol-tanned with degree of tannage 51. Chemical tests indicated that the increase in total sulfur and soluble nitrogen for the vegetable-tanned leathers were about twice that for the chrome-tanned leathers. All dressings failed to protect the vegetable-tanned leather effectively. Confirming the findings of previous investigators, it can be stated that deterioration is a hydrolytic process caused by the absorption of acid-producing compounds from the atmosphere.