Four leathers, chrome-tanned, vegetable tanned, semichrome, and chrome retan, were stored in closed jars over water for periods of up to 72 weeks at 40°C or 15 weeks at 60°C and tested at intervals for buckle tear load, shrinkage temperature, pH of water extract, and solublization of protein. All the leathers lost about 50per thousand of their strength at 60°C over water. The three leathers containing vegetable tan lost their fiber structure after 10 to 15 weeks and were very dark. The chrome leather, although weak, changed little in appearance. At 40°C the chrome-and vegetable-tanned leathers were relatively unaffected, compared to the combination-tanned leathers. Mold growth caused considerable losses in strength only in the chrome-tanned leather. The shrinkage temperature of the chrome-and vegetable-tanned leathers increased but that of the combination-tanned leathers decreased. Changes in the pH of water extract were small until the leathers lost their fiber structure, when increases from about pH 3 or 4 to pH 5 or 6 were observed in some cases. Only small increases in nitrogen soluble in 0. 1N sodium bicarbonate were found. The nitrogen soluble in 5M acetic acid increased more rapidly when the fiber structure began to be lost. Although the chrome-tanned leather became very weak in some instances, its soluble nitrogen was always low. In the grossly deteriorated leathers containing vegetable tan the volatile nitrogen in the extracts was very high, indicating that deamination of the amino acids of the collagen had occurred. The extractability of the chrome tan was reduced, suggesting that changes had occurred in the chrome complex.