Three chemical mechanisms recently developed for use in urban- and regional-scale oxidant models have been compared. Approximately 400 simulations were conducted using an atmospheric model to determine the degree to which the mechanisms yield comparable predictions of ozone and other oxidants. The simulations were carried out over a range of initial conditions for a number of scenarios representative of urban and rural environments. Sensitivity studies were conducted to assess the effect of varying parameters, such as the initial ratio of volatile organic compounds (VOC) to oxides of nitrogen (NOx), the composition of the VOC mixture, and the temperature. The mechanisms were found to yield nearly identical predictions for ozone, peroxyacetylnitrate, and nitric acid under most conditions. Differences, however, were noted in simulations with (1) high NOx levels, (2) high levels of aromatic hydrocarbons, and (3) low afternoon temperatures. Predictions of hydrogen peroxide were in good agreement except at high VOC/NOx ratios where differences of as much as a factor of 3 were found.