The destruction of methane by a non-thermal plasma in atmospheric pressure gas streams of nitrogen with variable amounts of added oxygen has been investigated. The identities and concentrations of the end-products are determined by on-line FTIR spectroscopy and the plasma chemistry is interpreted using kinetic modelling. For a deposited energy of 118 kJ m(-3), the destruction is 12% in nitrogen decreasing monotonically to 5% in air. The major end-products are HCN and NH3 in nitrogen and CO, CO2, N2O, NO and NO2 for gas streams containing oxygen. The chemistry in pure nitrogen is predominantly due to reactions of electronically-excited nitrogen atoms, N(D-2). The addition of oxygen converts the excited state nitrogen into nitrogen oxides reducing the methane destruction which then arises by O and OH reactions yielding CO and, to a lesser extent, CO2. The modelling correctly predicts the magnitude of the methane destruction as a function of added oxygen and the concentrations of the end-products for processing in both nitrogen and air.