Monitoring of surface ozone over southern Africa has shown that ambient concentrations often exceed a threshold of 40 ppb at which damage to vegetation by ozone could be expected. The Cross Border Air Pollution Assessment Project (CAPIA) was therefore established to assess the potential impacts of ozone on maize, a staple food crop, in five southern African countries. Keywords: Photochemical modelling, CAMx, Mesoscale model 5, CAPIA, Biogenic emissions, Anthropogenic emissions.