Abstract Computer simulations of the homogeneous, gas phase chemical reactions which occur in the plume of a coal-fired power plant were conducted in an effort to understand the influence of various environmental parameters on the production of secondary pollutants. Input data for the model were selected to reproduce the dilution of a plume from a medium-sized power plant. The environmental conditions chosen were characteristic of those found during mid-August in the south-eastern United States. Under most conditions examined, it was found that hydroxyl radicals were the most important species in the homogeneous conversion of stack gases into secondary pollutants. Other free radicals, such as HO 2 and CH 3O 2, exceeded the contribution of HO radicals only when high background hydrocarbon concentrations are used. The conversion rates calculated for the oxidation of SO 2 to SO 4 2− in these plumes were consistent with those determined experimentally. The concentrations and relative proportions of NO x (from the power plant) and reactive hydrocarbons (from the background air) determine, to a large extent, the plume reactivity. Free radical production is suppressed during the initial stages of dilution due to the high NO x levels. Significant dilution is required before a suitable mix is attained which can sustain the free radical chain processes common to smog chemistry. In most cases, the free radical concentrations were found to pass through maxima and return to background levels. Under typical summertime conditions, the hydroxyl radical concentration was found to reach a maximum at a HC NO x ratio of approximately 20. The meteorological conditions, as well as the ambient and stack gas concentrations, determine the plume travel time at which the hydroxyl and other free radical concentration maximum occurs. The model also predicts the presence of ozone ‘bulges’ under a wide variety of environmental and plant operational conditions.