Two open-top chamber experiments were carried out at a continental and a coastal location in Spain to test the ozone sensitivity of spinach (Spinacia oleracea L.) and Swiss chard (Beta vulgaris L. var. cycla) under Mediterranean conditions. Three cultivars of spinach and two cultivars of Swiss chard currently grown in commercial fields were screened for ozone tolerance in terms of marketable yield. Different physiological parameters were measured to identify traits related with this tolerance and ozone effects on yield quality.All the spinach and Swiss chard cultivars tested in this study were sensitive to ozone in terms of marketable biomass yield. The commercial biomass decreased on average between 13.8 and 17.9% in spinach and between 0 and 20.1% in Swiss chard at a continental and a coastal site, respectively, after two months of ozone exposure under ambient plus 40ppb of ozone concentration compared to charcoal-filtered air. Ozone effects on the marketable yield were associated with the appearance of leaf visible injury and by reductions in the photosynthetic activity of the leaves.Swiss chard cultivars were generally more ozone tolerant than spinach cultivars. However, the ozone sensitivity differed between cultivars of the same species and effects were more pronounced at the coastal site, where environmental growing conditions favoured plant growth more than at the continental site.The physiological traits analysed (photosynthesis and gas exchange parameters and ascorbate concentration and redox status) were not related to differences in ozone tolerance between cultivars. Ozone effects on leaf nitrogen content and greenness reflected the impact of ozone on yield quality parameters for leafy vegetables, such as leaf nitrate concentration and the marketable value.These results show that leafy vegetables can be more sensitive to ozone than other horticultural crops like tomato, which is used as a representative crop in risk assessments of ozone effects on horticultural production in Europe.