Concentrations of H2O2, organic peroxides, and other photochemically generated species were calculated using a zero-dimensional box model. The range of emission rates considered centered on values that are representative of conditions in a moderately polluted region such as the eastern United States. Solar intensity, temperature, humidity, and mixing layer height were varied to simulate changing conditions over a yearly cycle. Predicted concentrations of H2O2 have a pronounced seasonal dependence similar to that observed. Evidence is presented that the seasonal behavior of peroxide concentration is due to a transition between two qualitatively different photochemical states of the atmosphere. These states are called the low and high NOx regimes according to whether radical production is greater or less than the NOx emission rate. In the low NOx regime, more radicals are formed than can be accommodated by reaction with NOx, and the ''excess'' radicals are removed by bimolecular radical--radical reactions leading in large part to peroxide. In this regime, peroxide formation is nearly proportional to the difference between radical source strength and the NOx emission rate and peroxide concentration scales with the factors affecting radical source strength (e.g., water vapor and photolysis rate constants). Radical production in the high NOx regime is insufficient to react with and remove all NOx. Consequences include the suppression of peroxide formation, a reduction in the oxidizing capacity of the atmosphere, and high concentrations of primary pollutants. ¿ American Geophysical Union 1991