The mechanisms of nitrogen incorporation in diamond are still an unsolved riddle. This is mainly due to the complexity of the processes involved as they not only depend on empirical parameters (e.g. vessel pressure, substrate temperature, the gas phase composition, type and concentration of the nitrogen containing compound used), but also on the plasma chemistry and the surface chemical reactions. In this study, small quantities (ppm range) of diatomic nitrogen are added to a conventional hydrogen-methane feed gas mixture in order to investigate the effect of nitrogen incorporation in diamond films prepared by microwave plasma assisted chemical vapour deposition (CVD). Optical emission spectroscopy (OES) is used to survey the plasma composition during deposition. The intensities of the CN, CH and C-2 emitting radicals and the Balmer atomic hydrogen emission lines are correlated to the Raman film quality and to the nitrogen content in the film measured by secondary ion mass spectrometry (SIMS).