Gas-phase chemistry provides important oxidants and gaseous precursors for secondary aerosol formation. Differences in gas-phase chemical mechanisms may lead to differences in aerosol predictions. In this work, the role of gas-phase chemical mechanisms in aerosol predictions will be studied using the NOAA Weather Research and Forecast Model with Chemistry (WRF/Chem). Two gas-phase chemical mechanisms are being coupled with the Model of Aerosol Dynamics, Reaction, Ionization, and Dissolution (MADRID) in WRF/Chem: the 2005 version of Carbon Bond Mechanism IV (CBM-IV) (CB05) and the CBM version Z (CBM-Z). While both mechanisms are a variant of CBM-IV, there are several differences between CB05 and CBM-Z mechanisms. For example, compared with CB05, CBM-Z does not include extended inorganic reaction set, explicit acetaldehyde, and lumped terpene chemistry. The WRF/Chem simulations with the same aerosol module but different gas-phase mechanisms will be conducted over the continental U.S. for July 2001. Simulated gaseous and aerosol concentrations with different gas-phase mechanisms will be compared and evaluated against available surface observations and satellite column measurements (e.g., carbon monoxide from Measurements of Pollution in the Troposphere (MOPITT), nitrogen oxides from Global Ozone Monitoring Experiment (GOME), and tropospheric ozone from the Total Ozone Mapping Spectrometer (TOMS), the solar Backscattered Ultravoilet (SBUV) instruments). Likely causes for discrepancies between simulations and observations as well as the sensitivity of aerosol predictions to gas-phase chemical mechanisms will be examined.