Recent investigations have highlighted the relative importance of the winter season for emissions of N 2 O from boreal soils. However, our understanding of the processes and environmental controls regulating these emissions is fragmentary. Therefore, we investigated the potential for, and relative importance of, N 2 O formation at temperatures below 0 o C in laboratory experiments involving incubations of a Swedish boreal forest soil. Our results show that frozen soils have a high potential for N 2 O formation and subsequent emission. Net N 2 O production rates at -4 o C equaled those observed at +10 to +15 o C at moisture contents >60% of the soil's water-holding capacity. The source of this N 2 O was found to be denitrification occurring in anoxic microsites in the frozen soil and temperature per se did not control the denitrification rates at temperatures around 0 o C. Furthermore, both net nitrogenmineralisation and nitrification were observed in the frozen soil samples. Based on these findings we propose a conceptual model for the temperature response of N 2 O formation in soils at low temperatures.