Thirteen different lots of vegetable-tanned leather were prepared, treated with varying amounts of sulfuric acid, tested for tensile strength, and placed in storage for two years, after which strength test were again made. The loss in strength on aging was used as measure of the deterioration, and the results are presented in a series of graphs as a function of the initial pH of the leather. It is shown that all of the leathers started to deteriorate at or near pH 3 regardless of the kind of vegetable-tanning material used, the degree of tannage, the conditions, and the pH of the original leather before being treated with acid. The hydrolysis of the leathers, as measured by determining the nitrogen extractable with cold water, also increased sharply near pH 3. The results strongly point whatever the combination of circumstances may be which causes it to have this critical value. Serious consideration of the value of a pH measurement for indicating the harmful acidity of leather is advocated, the limiting values for different leathers to be governed by the length of service expected and the conditions of use.