In mires, which occupy large areas of the boreal region, net ecosystem CO2 exchange ( NEE) rates vary significantly over various timescales. In order to examine the effect of one of the most influencing variables, the water table depth, on NEE the general ecosystem model GUESS-ROMUL was modified to predict mire daily CO2 exchange rates. A simulation was conducted for a lawn, the most common microtopographical feature of boreal oligotrophic minerotrophic mires. The results were validated against eddy covariance CO2 flux measurements from Degero Stormyr, northern Sweden, obtained during the period 2001 - 2003. Both measurements and model simulations revealed that CO2 uptake was clearly controlled by interactions between water table depth and temperature. Maximum uptake occurred when the water table level was between 10 and 20 cm and the air temperature was above 15 degrees C. When the water table was higher, the CO2 uptake rate was lower, owing to reduced rates of photosynthetic carbon fixation. When the water table was lower, NEE decreased owing to the increased rate of decomposition of organic matter. When the water table level was between 10 and 20 cm, the NEE was quite stable and relatively insensitive to both changes within this range and any air temperature changes above + 15 degrees C. The optimal water table level range for NEE corresponds to that characteristic of mire lawn plant communities, indicating that the annual NEE will not change dramatically if climatic conditions remain within the optimal range for the current plant community.