A low pressure recombining Ar plasma to which mixtures of N2 and O2 were added has been studied to explore the relevance of surface related processes for the total chemistry. The abundances of the stable molecules N2, O2, NO, N2O and NO2 have been measured by means of a combination of infrared tunable diode laser absorption spectroscopy and mass spectrometry. A gas phase chemical kinetics model was developed in CHEMKIN to investigate the contribution of homogeneous interactions to the conversion of the feedstock gases N2 and O2. At a partial pressure of N2 plus O2 less than 8 Pa, significant discrepancies between measured and calculated concentrations of N2O and NO2 were observed, indicating that heterogeneous processes are dominating the chemistry in this pressure range. At a partial pressure of N2 plus O2 higher than 40 Pa and a relatively high fraction of admixed O2 we observed a fair agreement between measured and calculated concentrations of NO molecules, indicating that homogeneous processes (notably N atom induced) are more dominant than heterogeneous processes.