Young trees of European beech (Fagus sylvatica) acclimated for one growing season to ambient (c. 367 μl l−1) or elevated CO2 levels (c. 660 μl l−1) were exposed during the subsequent year to combinations of the same CO2 regimes and ambient or twice-ambient ozone (O3) levels (generated from the database of a rural site). By the end of June, before the development of macroscopic leaf injury, the raised O3 levels had not affected the light and dark reactions of photosynthesis. However, acclimation to elevated CO2 had resulted in lowered chlorophyll and nitrogen concentrations, whereas photosynthetic performance, examined over a wide range of parameters from light and dark reactions, remained unchanged or showed only slight reductions (e.g. apparent electron transport rate, ETR; apparent quantum yield of CO2 gas exchange, ΦCO2; apparent carboxylation efficiency, CE; and photosynthetic capacity at light and CO2 saturation, PC). In August, after the appearance of leaf necroses, plants grown under ambient CO2 and twice-ambient O3 conditions declined in both the photosynthetic light reactions (optimum electron quantum yield, Fv/Fm, non-photochemical energy quenching, NPQ, reduction state of QA, apparent electron quantum yield, ΦPSII, maximum electron transport rates) and the dark reactions as reflected by CE, ΦCO2, as well as the maximum CO2 uptake rate (i.e. PC). CE, ΦCO2 and PC were reduced by c. 75, 40 and 75%, respectively, relative to plants exposed to ambient CO2 and O3 levels. By contrast, plants exposed to twice-ambient O3 and elevated CO2 levels maintained a photosynthetic performance similar to individuals grown either under ambient CO2 and ambient O3, or elevated CO2 and ambient O3 conditions. The long-term exposure to elevated CO2 therefore tended to counteract adverse chronic effects of enhanced O3 levels on photosynthesis. Possible reasons for this compensatory effect in F. sylvatica are discussed.