The negative ions formed in N2O and N2O and N2O + O2 mixtures have been studied in the gas phase using a drift tube and mass filter. Gas pressures were in the Torr range, and reduced fields were varied between 10–17 and 10–15 V cm2 molecule–1. The observed ion spectrum was found to be governed by the following reaction, with their associated thermal rate constants: e + N2O → N2+ O–, k1= 4 × 10–15 cm3 molecule–1 s–1, O–+ N2O → NO–+ NO, k3=(1.95 ± 0.06)× 10–10 cm3 molecule–1 s–1, NO–+ N2O → NO–2+ N2, k4=(2.8 ± 0.2)× 10–14 cm3 molecule–1 s–1, NO–+ 2N2O → N3O–2+ N2O, k5=(8.5 ± 1.5)× 10–30 cm6 molecule–2 s–1, O–+ 2N2O → N2O–2+ N2O, k6=(4.2 ± 0.5)× 10–29 cm6 molecule–2 s–1, NO–+ N2O → N2O + NO + e, k11=(6.0 ± 1.0)× 10–12 cm3 molecule–1 s–1, O–2+ N2O → O–3+ N2, k10 < 10–12 cm3 molecule–1 s–1. The rates of reaction, (5), (6) and (11) were weak functions of reduced field. In no experiment was any evidence found for the existence of a long-lived N2O– ion.