The nature of two summertime photooxidant pollution episodes over Paris is investigated by means of adjoint modeling. The first episode (July 12, 1994) is characteristic of a 'local production' episode, while the second (August 12, 1997) is mostly due to advection into the urban area of high concentrations of ozone. We calculate the sensitivities to emissions of individual primary species and to reaction rate coefficients using the adjoint of a simplified multibox model representing the urban and suburban areas of the city of Paris. Sensitivity to emissions demonstrates that the first episode is essentially sensitive to volatile organic compounds (VOC)/NOx emissions, while the second is sensitive to NOx emissions. These results also point out the two important emission activity categories: traffic and solvent use, the second one being more sensitive than the first one. Sensitivity to reaction rates indicates that oxidant production is only sensitive to a few reactions. The high sensitivity of photochemical pollutants peaks to particular chemical reactions points out the necessity to refine the knowledge of their reaction rates. ¿ 2000 American Geophysical Union