If we consider a cumyl peroxide vulcanizate, prepared from deproteinized crepe and from which all vulcanization residues have been extracted after cure, the variation of the overall quantum yield for chain scissions ϕ, as a function of wavelength, deduced from relaxation measurements in the air at 30° C, presents several discontinuities. At the beginning of the solar ultraviolet, ϕ equals 5.4×10−2 near 330 m/µ, it then presents a minimum value of 3×10−2, at about 340 mµ, before reaching a very pointed maximum of 5.3×10−2 at 355 mµ. Afterwards, it vanishes at the start of the visible radiation. It should be noted that ϕ in the ultraviolet region is of the same order of magnitude as the corresponding value found in the case of polymethyl methacrylate (1.2×10−2). In investigating the different parts of the spectrum in which the dissociation of chemical bonds present in the isoprene unit is energetically possible two assumptions seem justified: one, the high value of ϕ near 330 mµ may be due to photolysis of the main chain C—C bonds adjacent to the double bond; this will be manifest directly as a random breaking in the polymer chain, and two, the maximum of ϕ at 355 mµ presumably results from the photochemical dissociation of the α-methylenic C—H bonds. The radicals so formed initiate an autoxidation process, producing in the termination step oxidative cleavage of the chain. On the other hand, sulfur vulcanizates prepared from deproteinized crepe, vulcanized with mercaptobenzothiazole and sulfur and extracted after cure, contains some residues of vulcanization, namely zinc oxide and zinc sulfide, which are not extracted by the solvents employed. In this case, a determination of the overall quantum yield for scissions will not be adequate for theoretical purposes. From the practical point of view, an investigation of the influence of free substances present in different vulcanizates shows, once again, that non-rubber constituents display a protective effect. Otherwise, the action of vulcanization residues clearly depends upon their nature. Mercaptobenzothiazole and sulfur considerably slow down degradation by solar ultraviolet; zinc oxide protects only against radiation below 370 mµ, but it presents a sensitizing effect in the near visible. As a result, the most harmful rays for the greatest part of pure gum vulcanizates are found, not at the beginning of the ultraviolet sunlight, but in the region of 400 mµ.