Certain properties of the paper in twelve U.S. Geological Survey maps printed between 1918 and 1971 were measured and the data obtained were used to estimate the condition of map collections. A statistical analysis of the present-day M.I.T. folding endurance data suggests that the probable useful life of U.S.G.S. maps is approximately 100 years. These findings imply that U.S. G.S. maps are aging at rates similar to the deterioration rates of most 20th century book papers. The extension of map life obtained through air-conditioning, e.g., by storage at reduced relative humidities and temperatures, is discussed. The experimental data presented indicate that both aqueous and non-aqueous deacidification treatments increase the potential life of most maps and that non-aqueous treatments can produce more protection against the development of harmful acidity in the future than aqueous treatments can produce.