A global uncertainty analysis of the Regional Acid Deposition Model version 2 chemical mechanism was conducted to estimate the effect of uncertainties in rate parameters and product yields on predicted concentrations of ozone and other secondary pollutants. Uncertainties were also estimated for changes in ozone due to 25% reductions in reactive organic gases (ROG), nitrogen oxides (NOx) or both. The analyses were performed for single-day box model simulations of summertime smog episodes, with uncertainties propagated using a Latin hypercube sampling scheme. The resulting uncertainties in peak ozone concentrations range from about 20 to 50% (¿1&sgr; relative to the mean). Uncertainties in predicted concentrations range from 15 to 30% for HNO3, 20 to 30% for HCHO, and 40 to 70% for peroxyacetyl nitrate (PAN). Except with very low concentrations, uncertainties in H2O2 range from 30 to 45%. For the cases studied the choice between ROG or NOx reductions as most effective for lowering ozone concentrations is insensitive to uncertainties in chemical parameters. Product concentrations are most strongly affected by uncertainties in rate parameters for PAN chemistry, HNO3 formation, aromatics oxidation, the reaction of HO2+NO, and photolysis of HCHO, NO2, and O3, and uncertainties in peroxy radical yields from aromatics and olefins oxidation. ¿ American Geophysical Union 1996