The sensitivity of tropospheric O3 to aqueous HOx (OH+HO2) phase chemistry in clouds is examined using photochemical model calculations of the O3 production efficiency per unit NOx and the chemical lifetime of O3, combined with estimates for the residence time of air in clouds. It is found that the maximum perturbation to O3 from cloud chemistry in the tropics and midlatitudes summer is less than 3%. This result is supported by calculations using a three-dimensional, continental-scale model for North America.¿ 1997 American Geophysical Union