Confidence in regional estimates of N2O emissions used in national greenhouse gas inventories could be improved by using mathematical models of the biological and physical processes by which these emissions are known to be controlled. However these models must first be rigorously tested against field measurements of N2O fluxes under well documented site conditions. Spring thaw is an active period of N2O emission in northern ecosystems and thus presents conditions well suited to model testing. The mathematical model ecosys, in which the biological and physical processes that control N2O emissions are explicitly represented, was tested against N2O and CO2 fluxes measured continuously during winter and spring thaw using gradient and eddy covariance techniques. In the model, ice formation at the soil surface constrained soil-atmosphere gas exchange during the winter, causing low soil O2 concentrations and consequent accumulation of denitrification products in the soil profile. The removal of this constraint to gas exchange during spring thaw caused episodic emissions of N2O and CO2, the timing and intensities of which were similar to those measured in the field. Temporal variation in these emissions, both simulated and measured, was high, with those of N2O ranging from near zero to as much as 0.8 mgNm-2h-1 within a few hours. Such variation should be accounted for in ecosystem models used for temporal integration of N2O fluxes when making long-term estimates of N2O emissions. ¿ 1999 American Geophysical Union