Degradation processes driven by UV exposure, and manifested for example as polymer gloss loss or coating failure, are generally accelerated at elevated temperature, or conversely, their rates are reduced at lower temperature. In a weathering environment comprising IR irradiance, IR transmitting black resin tends to be cooler than an otherwise comparable sample of IR absorbing black resin. Accordingly, slower UV-driven degradation, and longer weathering lifetime, is expected for IR transmitting black resin relative to IR absorbing black resin, commensurate with their temperature difference in a given weathering environment and the sensitivity of the degradation process to temperature. This paper reports temperature differences between IR absorbing and IR transmitting black resins as measured in two types of filtered xenon-arc accelerated weathering instruments for a range of black panel and chamber temperatures, several configurations of light and dark filler coupons, and two commercial filter combinations that differ in their IR transmittance. These temperature differences under xenon arc are compared with the temperature difference between the same samples measured outdoors in southeast Michigan. The temperature difference under the various xenon arc treatments generally exceeds the difference outdoors for dry, high irradiance conditions, and does so invariably for effective temperatures, which account for time-varying temperature and irradiance over the period of an instrument cycle or the period of a year in Miami. Thus, the difference in UV exposure to failure in the accelerated weathering instruments overstates the expected difference in exposure to failure outdoors. The degree of overstatement is relatively sensitive to black panel and chamber temperatures, filter combination and sample elevation within a sample holder, is relatively insensitive to the configuration of filler coupons, and can be estimated for cases of practical interest where the degradation process is characterized by an activation energy.