This report represents a study of the accelerated aging of chemically modified handsheets in air and nitrogen at 90° C and 50 percent or 0 percent relative humidity. The handsheets were prepared from a commercial wood pulp that had been either (a) untreated, (b) deashed with 0.1 N HC1, or (c) subjected to ion exchange treatment, first with calcium acetate and then with aluminum sulfate. The folding endurance, tearing resistance, and bursting strength all decrease rapidly as the fiber strength of aluminum treated or deashed paper decreases with aging in humid gases. Decreases in fold, tear, burst, and tensile strength occur more rapidly in humid air than in humid nitrogen, indicating that oxidation is responsible for some of the changes in humid air. The strength of aluminum treated paper decreases in dry gases appreciably more rapidly than that of deashed paper, while untreated paper retains most of its initial strength throughout dry aging. -- AATA