The high-arctic environment is an environment where the consequences of global warming may be significant. In this paper we report on findings on carbon dioxide and water vapour fluxes above a sedge-dominated fen at Zackenberg (74°28′N, 20°34′ W) in The National Park of North and East Greenland. Eddy covariance measurements were initiated at the start of the growing season and terminated shortly before its end lasting 45 days. The net CO2 flux during daytime reaches a high of 10 μmol m–2s–1, and around the summer solstice, net CO2 assimilation occurred at midnight, resulting in net carbon gain during the night. The measured carbon dioxide fluxes compare well to estimates based on the photosynthesis model byCollatz et al. (1991). The total growing-season net ecosystem CO2 exchange was estimated to be 96 g C m–2 based on the carbon dioxide model and micrometeorological data. Finally, the combined CO2 assimilation and soil respiration models are used for examining the dependence of the carbon dioxide budget on temperature. The ecosystem is found to function optimally given the present temperature conditions whereas either an increase or a decrease in temperature would reduce the ecosystem CO2 accumulation. An increase in temperature by 5 °C would turn the ecosystem into a carbon dioxide source.