First-order sensitivity and uncertainty analysis methods have been applied to the RADM2 mechanism, the second generation gas-phase chemical mechanism included in the Regional Acid Deposition Model. The direct decoupled method (DDM) was used to evaluate the local sensitivity of product concentrations of O3, HCHO, H2O2, PAN, and HNO3 to values of 157 rate constants and 126 stoichiometric coefficients. The sensitivity analysis results were combined with estimates of the uncertainty in each parameter in the mechanism to produce a local measure of its contribution to the uncertainty in the outputs. Several different sets of simulation conditions were used, representing summertime surface conditions for urban and nonurban areas. The analysis identified the most influential rate parameters to be those for PAN chemistry, formation of HNO3, and photolysis of HCHO, NO2, O3, and the unknown product (DCB) of aromatics oxidation. Rate parameters for the conversion of NO to NO2, such as O3+NO, HO2+NO and organic radical+NO, and the product yields of organic peroxy radical (XYLP) in the reaction of XYL+HO (XYL represents xylene and more reactive aromatics) and DCB in the reaction XYLP+NO are also relatively influential. ¿ American Geophysical Union 1995