The reactions of organic peroxy radicals with NO, HO2, organic peroxy radicals, and NO3 along with the formation and decomposition of peroxyacetyl nitrate are all important for the modeling of atmospheric chemistry. Recent laboratory measurements of the rate constants for peroxy radical-peroxy radical reactions show that there are large differences between these rate constants. We present methods of estimating organic peroxy radical self-reaction rate constants from an empirical expression. These self-reaction rate constants may be used to estimate the rates of peroxy radical cross reactions. There are also new data available on the rate constants for peroxyacetyl nitrate formation, decomposition, the reaction of acetyl peroxy radicals and other organic peroxy radicals with NO, and the reactions of NO3 with organic peroxy radicals. To estimate the importance of these reactions, these new data along with revisions to the product yields for organic peroxy radical-organic peroxy radical reactions were implemented in the mechanism of Stockwell et al. <1990>. The revised mechanism yields significantly different concentrations of peroxyacetyl nitrate, higher organic hydroperoxides and peroxyacetic acid concentrations. Nighttime concentrations of organic peroxy radicals, HO2, HO, and NO3 are also strongly affected by organic peroxy radical-organic peroxy radical reactions and the reactions of organic peroxy radicals with NO3. Our results suggest that the reactions of organic peroxy radicals with NO3 are more important than organic peroxy radical-organic peroxy radical reactions in the nighttime atmosphere. ¿ American Geophysical Union 1996