Studies of the deterioration of leather under warm moist conditions have mainly been concerned with heavy upper leathers of high grease content. In the present work a survey of the behavior of a selection of commercial leathers of various types has been made in order to form a general background to a program of research on the mechanism of the deterioration of leather. The leathers were stored over water at 40° C. for 6 months, and losses in strength and changes in crackiness, shrinkage, temperature, and pH of water extract were determined. Chrome leathers were the most resistant to the action of heat and moisture and, with the exception of the one sole leather tested, lost little or no strength. Some of the vegetable-tanned leathers also lost relatively little strength but darkened in color and tended to become rather cracky. The resistance of the majority of the semichrome and chrome retan leathers was poor, and losses of strength of 80per thousand occurred in some cases. The shrinkage temperature of most of these leathers fell by 10° or more during the storage. The possible causes of the low resistance of these leathers are considered, and their bearing on the behavior of these leathers in wear are discussed. Limited tests on the same leathers stored over water at 20° C. or in air at 40° C. indicated that little deterioration occurs under these conditions, emphasizing that it is the combined action of heat and moisture which is deleterious.