A sensitivity analysis of a comprehensive mixed-phase chemical mechanism is conducted under a variety of atmospheric conditions. The local sensitivities of gas and aqueous phase species concentrations with respect to a variety of model parameters are calculated using the novel automatic differentiation ADIFOR tool. The main chemical reaction pathways in all phases, interfacial mass transfer processes, and ambient physical parameters that affect tropospheric O3 formation and O3-precursor relations under all modeled conditions are identified and analyzed. The results show that the presence of clouds not only reduces many gas phase species concentrations and the total oxidizing capacity but alters O3-precursor relations. Decreases in gas phase concentrations and photochemical formation rates of O3 can be up to 9% and 100%, respectively, depending on the preexisting atmospheric conditions. The decrease in O3 formation is primarily caused by the aqueous phase reactions of O2- with dissolved HO2 and O3 under most cloudy conditions. ¿ 1998 American Geophysical Union