<p>We measured CO<sub>2</sub> and CH<sub>4</sub> fluxes using chambers and eddy covariance (only CO<sub>2</sub>) from a moist moss tundra in Svalbard. The average net ecosystem exchange (NEE) during the summer (June–August) was −0.40 g C m<sup>−2</sup> day<sup>−1</sup> or −37 g C m<sup>−2</sup> for the whole summer. Including spring and autumn periods the NEE was reduced to −6.8 g C m<sup>−2</sup> and the annual NEE became positive, 24.7 gC m<sup>−2</sup> due to the losses during the winter. The CH<sub>4</sub> flux during the summer period showed a large spatial and temporal variability. The mean value of all 214 samples was 0.000511 ± 0.000315 µmol m<sup>−2</sup>s<sup>−1</sup> which corresponds to a growing season estimate of 0.04 to 0.16 g CH<sub>4</sub> m<sup>−2</sup>. We find that this moss tundra emits about 94–100 g CO<sub>2</sub>-equivalents m<sup>−2</sup> yr<sup>−1</sup> of which CH<sub>4</sub> is responsible for 3.5–9.3 % using GWP<sub>100</sub> of 27.9 respectively GWP<sub>20</sub>.</p> <p>Air temperature, soil moisture and greenness index contributed significantly to explain the variation in ecosystem respiration (R<sub>eco</sub>) while active layer depth, soil moisture and greenness index were the variables that best explained CH<sub>4</sub> emissions. Estimate of temperature sensitivity of R<sub>eco</sub> and gross primary productivity showed that a modest increase in air temperature of 1 degree did not significantly change the NEE during the growing season but that the annual NEE would be even more positive adding another 8.5 g C m<sup>−2</sup> to the atmosphere. We tentatively suggest that the warming of the Arctic that has already taken place is partly responsible for the fact that the moist moss tundra now is a source of CO<sub>2</sub> to the atmosphere.</p>