A commercial leather and leathers tanned with sumac, chestnut, quebracho, cutch, and a blend of chestnut, sulfited quebracho, and cutch, were heated to temperatures from 67° to 130°C in helium, in air, and in oxygen, at atmospheric pressure in the absence of water vapor. The effect of varying the concentration of water vapor was studied with chestnut and quebracho leathers at 100°C. The loss of tensile strength and the increase in the nitrogenous materials extractable in 0.1 N sodium carbonate solution were used as measures of the deterioration, the former being regarded as the more reliable. Raising the temperature increased the deterioration under all circumstances. In air, deterioration was more rapid than in the inert gas (helium), and in oxygen it was more rapid than in air. The logarithm of the fractional deterioration of the dry leather increased linearly with the duration of heating in air. The effect of humidity was small for samples having a pH greater than 3 but increased rapidly at lower values of the pH. The most important features of an accelerated-aging test for leather are an elevated temperature and an oxidizing atmosphere containing moisture.