A state-of-the-art gas phase chemical mechanism for modeling atmospheric chemistry on a regional scale is presented. The second generation Regional acid Deposition Model (RADM2) gas phase chemical mechanism, like its predecessor RADM1, is highly nonlinear, since predicted ozone, sulfate, nitric acid and hydrogen peroxide concentrations are complicated functions of NOx and nonmethane hydrocarbon concentrations. The RADM2 chemical mechanism is an upgrade of RADM1 in that (1) three classes of higher alkanes are used instead of one, (2) a more detailed treatment of aromatic chemistry is used, (3) the two higher alkene classes now represent internal and terminal alkenes, (4) ketones and dicarbonyl species are treated as classes distinct from aldehydes, (5) isoprene is now included as an explicit species, and (6) there is a more detailed treatment of peroxy radical-peroxy radical reactions. As a result of these improvements the RADM2 mechanism simulates the concentrations of peroxyacetyl nitrate, HNO3, and H2O2 under a wide variety of environmental conditions. Comparisons of RADM2 mechanism with the RADM1 mechanism predictions and selected environmental chamber experimental results indicate that for typical atmospheric conditions, both mechanisms reliably predict O3, sulfate and nitric acid concentrations. The RADM2 mechanism gives lower and presumably more realistic predictions of H2O2 because of its more detailed treatment of peroxy radical-peroxy radical reactions. ¿ American Geophysical Union 1990