Abstract (1) Using the rubber-sulfur mixture and the mixture containing accelerator and zinc oxide, the relation between the maximum state of acetone extracts and the degree of oxidation was studied quantitatively. (2) In carrying out the above experiment, the samples previously extracted were used in order to accelerate the oxidation and to eliminate the influence of the free sulfur and rubber resin, etc. The aging was conducted at 70° C. for the rubber-sulfur stock and 70° C. and 90° C. for the stock containing accelerator and mineral filler and the increase of weight and the amount of the acetone extract were measured at definite intervals. The maximum state occurred in both cases, but for the case containing accelerator and filler the occurrence was comparatively slow. The decrease of acetone extract after the maximum state was shown to be due to partial formation of some acetone-insoluble products in the aged vulcanized rubber when oxidation proceeds considerably. (3) From the elementary analysis made on the aged samples from which acetone-soluble oxidation products were previously removed, it was shown that the oxygen content in these samples increased as aging proceeded.