Effects of elevated tropospheric ozone concentration on the CO2 exchange of peatland microcosms and the photosynthetic capacity of the dominating sedge, Eriophorum vaginatum, were studied in a four-year open-field experiment. The net ecosystem CO2 exchange and the dark respiration rate of the microcosms were measured with the closed chamber method. The CO2 assimilation rate and chlorophyll fluorescence (maximal photochemical efficiency of PSII, Fv/Fm) of E. vaginatum leaves were also measured. The gross photosynthesis rate of the microcosms was transiently decreased by ozone exposure during the first year. During the fourth year, the gross photosynthesis and dark respiration rate were both slightly increased by ozone exposure but this was due to the increased density of sedge leaves and no difference was found in Fv/Fm. In overall, chronic ozone exposure had only slight effect on the CO2 exchange of the peatland microcosms.