The reaction channels and rate coefficients for the reactions of O2+ and O− ions with NH3 have been determined at 21.4 °C in a flowing afterglow of pure oxygen and a mixture of helium and oxygen. Charge-transfer was found to be the dominant O2+ reaction channel whereas hydrogen atom abstraction was found to be the dominant O− reaction channel. The secondary NH3+ ions were found to react further with NH3 by hydrogen abstraction to yield NH4+ ions, which readily associated with NH3 to yield cluster ions. The further reaction of secondary OH− ions with NH3 was not observed suggesting that the electron affinity of NH2 may be less than 1.10 eV.