Gaseous N emissions arising from biological denitrification in soils is of economic and environmental consequence. However, the biological and physical processes controlling emissions are complex, and, therefore, mathematical simulation is useful in understanding them. A mathematical model was constructed from a hypothesis based on preferential reduction of O2, followed by NO3−and then N2O during microbial oxidation of carbon substrates. The hypothesis allowed the model to reproduce the time course of N2O and N2production and consumption during anaerobic incubations of soil amended with 37.5 to 300 μg NO3−-N g−1soil. Biologically based models of denitrification, coupled to physically based models of the soil environment, may allow greater insight into the dynamics of denitrification and N2O emission in the field.