A one-dimensional box model has been formulated to describe the ozone budget and HOx photochemistry in the marine boundary layer. The model includes a simple description of vertical exchange with the free troposphere and the ocean surface, and a photochemical scheme including oxidation of CO and CH4. Model calculations are compared with data collected during the summer season at two midlatitude coastal sites in the northern and southern hemispheres (Mace Head, Ireland, and Cape Grim, Tasmania). Results using small, prescribed NOx concentrations gave compensation points, where ozone loss by photolysis and physical removal is balanced by its production via NOx chemistry, of ~30 ppt and ~15 ppt at the two sites, respectively, in line with conclusions from the observational data. Changes in the dependence of peroxy radical concentrations on solar intensity were also in line with observations. The simple model involving CO and CH4 chemistry should be adequate for defining oxidizing capacity in the unpolluted marine boundary layer. ¿ 1999 American Geophysical Union