Organic peroxy radicals are important intermediates in the atmospheric photooxidation of hydrocarbons. Laboratory studies indicate that these radicals react among themselves (permutation reactions) at rates which should compete with those for reaction with HO2 and NOx under some atmospheric conditions. Using the available laboratory data, we have developed a simplified method of representing the large number of atmospheric peroxy radical permutation reactions, and have included this in a detailed mechanism of hydrocarbon photooxidation. The potential importance of these reactions was studied for relatively low NOx conditions such as may be observed in the marine planetary boundary layer (PBL) and the Amazon PBL. The effects of the permutation reactions include suppression of organic peroxy radical concentrations, higher HO2 and H2O2 concentrations, shorter lifetimes for PAN and other peroxyacyl nitrates (with concomitant faster release of NOx), and production of substantial concentrations of alcohols and organic acids (including acetic acid). If the kinetic and mechanistic data used in the present study are at least roughly realistic, the peroxy radical reactions probably play an important role in the photochemistry of the clean troposphere. ¿ American Geophysical Union 1990