Abstract All SBR vulcanizates, when tested in a relaxed state, reacted with ozone of low concentrations to form a film of oxidized products which provided an effective barrier against further attack by gaseous ozone. SBR vulcanizates that were under stress, but which contained no effective antiozonant in their formulation, were readily attacked by ozone. The absorption of ozone appeared to begin with an initial rate of zero which was followed by rapidly increasing rates until a maximum had been reached, when ozone cracks were visible on the surface of the rubber. SBR vulcanizates that were under stress, but which contained an antiozonant, were protected from an attack by ozone to a degree that ranged from poor to excellent. The degree of protection depended (a) on the differential in the rates of reaction of the antiozonant and the rubber hydrocarbon with ozone, (b) on the initial concentration of the antiozonant on the surface of the vulcanizate which reacted with ozone to form a barrier of oxidized residues, and (c) on the rate of effusion of fresh antiozonant from within the rubber to the outer surface of the barrier. SBR vulcanizates containing a naphthenic processing oil as an extender were not so resistant to ozone as standard SBR vulcanizates. It is probable that the effectiveness of the antiozonants tested in these vulcanizates was reduced by their high solubility in the oil phase. SBR vulcanizates containing trioctyl phosphate as a plasticizer were vigorously attacked by ozone. It is possible that the gaseous ozone dissolved to some extent into the plasticizer phase, increasing the concentration of ozone in the area causing a more severe oxidation of the rubber. SBR vulcanizates that had been coated with an antiozonant by dipping the specimen several times into a solution of the antiozonant in a solvent were found to contain a higher concentration of the antioxonant directly on the surface of the vulcanizate than in the case where three parts of the antiozonant were added during vulcanization. A mechanism for the ozonization of SBR vulcanizates has been proposed which includes a possible mechanism for the protective action of antiozonants. A rate equation has been derived from this mechanism which was consistent with the experimental rate data.