We report the results of a 2-year study of effects of the elevated (current ambient plus 350 μmol CO2 mol−1) atmospheric CO2 concentration (Ca) on net ecosystem CO2 exchange (NEE) of a scrub–oak ecosystem. The measurements were made in open-top chambers (OTCs) modified to function as open gas-exchange systems. The OTCs enclosed samples of the ecosystem (ca. 10 m2 surface area) that had regenerated after a fire, 5 years before, in either current ambient or elevated Ca. Throughout the study, elevated Ca increased maximum NEE (NEEmax) and the apparent quantum yield of the NEE (φNEE) during the photoperiod. The magnitude of the stimulation of NEEmax, expressed per unit ground area, was seasonal, rising from 50% in the winter to 180% in the summer. The key to this stimulation was effects of elevated Ca, and their interaction with the seasonal changes in the environment, on ecosystem leaf area index, photosynthesis and respiration. The separation of these factors was difficult. When expressed per unit leaf area the stimulation of the NEEmax ranged from 7% to 60%, with the increase being dependent on increasing soil water content (Wsoil). At night, the CO2 effluxes from the ecosystem (NEEnight) were on an average 39% higher in elevated Ca. However, the increase varied between 6% and 64%, and had no clear seasonality. The partitioning of NEEnight into its belowground (Rbelow) and aboveground (Rabove) components was carried out in the winter only. A 35% and 27% stimulation of NEEnight in December 1999 and 2000, respectively, was largely due to a 26% and 28% stimulation of Rbelow in the respective periods, because Rbelow constituted ca. 87% of NEEnight. The 37% and 42% stimulation of Rabove in December 1999 and 2000, respectively, was less than the 65% and 80% stimulation of the aboveground biomass by elevated Ca at these times. An increase in the relative amount of the aboveground biomass in woody tissue, combined with a decrease in the specific rate of stem respiration of the dominant species Quercus myrtifolia in elevated Ca, was responsible for this effect. Throughout this study, elevated Ca had a greater effect on carbon uptake than on carbon loss, in terms of both the absolute flux and relative stimulation. Consequently, for this scrub–oak ecosystem carbon sequestration was greater in the elevated Ca during this 2-year study period.