One lot of quebracho tanned leather and one lot of chestnut tanned leather were treated to contain varying percentages of sulphuric acid. One half of the leather was aged at 70° F., and the remaining half at 110° F. Relative humidities of 65 per cent and 75 per cent, respectively, were maintained to keep the moisture content of the leathers approximately constant at both temperatures. The deterioration of the leathers, as determined by the loss in strength and also the amount of extractable nitrogen, was greater at the higher temperature. The effect increased with an acid content up to about 2 1/3 per cent for the quebracho leather and 1 1/2 per cent for the chestnut leather.