The reactions of ozone with alkenes have been shown recently to lead to the direct production of OH radicals. Organic peroxy radicals (RO2) probably accompany the production of OH. In this paper, we draw attention to the potential importance of these reactions in the primary production of HOx (HOx=OH, HO2 and RO2) radicals in various regions of the boundary layer. The reactions of ozone with anthropogenic alkenes are shown to be the most important source of HOx in many urban settings during the day and evening, and a significant source at night. The majority of HOx comes from trace quantities of alkenes with internal double bonds. Reaction of O3 with isoprene and terpenes can be an important source of HOx in forested regions; we show that these reactions are the dominant radical source in the late afternoon and into the night. This additional HOx source is expected to increase predicted OH concentrations compared to those calculated by models that do not include the O3-alkene source. ¿ American Geophysical Union 1996