A regional air quality model is used to assess the impact of inert organic nitrate formation on ground-level ozone in the eastern United States during summer. The chemical mechanism used is the Carbon Bond Mechanism 4 (CBM4), which is widely used by regulatory agencies in the United States in air quality modeling applications. Recently, modifications were made to the reaction mechanism involving the organic peroxy radicals which form inert organic nitrates without a critical scientific review of the effects of these changes. In this study, we demonstrate for the first time that the simulated large-scale distribution of ground-level ozone is extremely sensitive to these mechanism changes. Inclusion of radical-radical reactions involving the organic peroxy radicals suppresses inert organic nitrate formation, and leads to significant increases in nitrogen oxide levels over large parts of the model domain. As a consequence of increased rates of ozone photochemical production, ozone mixing ratios are enhanced by as much 10--25 ppbv when these additional radical termination pathways are considered in the model. ¿ 1997 American Geophysical Union