The digital print industry is working toward the goal of adopting improved test methods for image permanence. As part of this effort, the new test methods are being designed to isolate the environmental variables that impact image permanence. The benefits of this approach are that it simplifies the test method and test equipment while promoting test results that can be reproduced at other test laboratories. To understand which variables to isolate for a given test method, it is necessary to investigate a broad range of conditions which may affect the test results. These variables may not have been controlled in the past, are difficult to control, or may not accurately reflect real world conditions. In a prior paper, it was shown that airflow can affect the rate of paper yellowing in a thermal stability test used to simulate dark storage conditions.During that testing, additional sources of potential noise were identified that affected the test results. This paper focuses on additional thermal stability experiments that studied the influence of ozone exposure on the rate of paper yellowing under thermal test conditions. Moreover, due to the rapid yellowing of some microporous photo paper after exposure to ozone, it is also possible to compare the validity of an accelerated thermal test method in predicting results at ambient conditions. This research is part of ongoing work contributing to the development of standardized test methods for image permanence.