Leather tanned with chestnut-wood extract and with quebracho-wood extract was treated with sulphuric acid in amounts varying from 1/4 to 4 per cent. The tensile strength of these samples was determined immediately after the treatment with acid and the remaining portions were aged under conditions of 65 per cent relative humidity at 70° F. Selected groups of samples were tested for strength again after 2, 4, 6, 9, 12, and 24 months, and the change in tensile strength was considered a measure of the deterioration. Data are presented in series of graphs which show the influence of time, acid content, type of tanning material and the degree of tannage on the deterioration of the leather.