Abstract It has been previously shown by the authors that two separate and distinct factors are responsible for the changes which rubber compounds undergo when exposed outdoors : a light-energized oxidation and attack by ozone which is normally present in the atmosphere in minute concentration. The former acts independently of stress in the rubber, the latter only when the material is stretched. Since sunlight, ozone, and temperature vary hourly, daily, seasonally, and with locality, the overall result to the exposed material depends on the balance between the causative factors and is unique, since any given combination of such factors is not duplicated. It is, therefore, essential that the susceptibilities of a given compound to damage by these two factors be determined separately. The information thus furnished will then make it possible to estimate the durability of the compound under any given set of conditions. Damage by light-energized oxidation can be mitigated to any great extent only by physical protection from light, which can be partially achieved by incorporation of carbon black or ferric oxide. It is likely to be the major factor in the case of light-colored goods. Susceptibility can be readily gauged by measurement of the oxygen absorbed when the material is irradiated in air or oxygen under controlled conditions. The present obstacle to formulation of a standard test is the lack of a constant and reproducible light source which simulates sunlight. This article is principally concerned with the ozone factor. In the case of rubber goods containing appreciable loadings of carbon black, such as insulated cable jackets and hose, by far the most important cause of deterioration is the familiar cracking at stressed locations by the ozone of the atmosphere, popularly and erroneously referred to as “light cracking”.