The kinetics of GaAs nitridation using N-2 plasmas, both radio frequency and electron cyclotron resonance sources, is investigated using in situ and real time ellipsometry. A comparison of plasma nitridation with the more conventional NH3 thermal nitridation of GaAs is also reported. We report that all the GaAs nitridation processes are self-limiting yielding only very thin GaN layers. The dependence of GaN layer thickness on surface pretreatment, surface temperature and N atom density in the plasma is reported. Smooth and stoichiometric GaN layers are formed at T < 600 degrees C, whereas nitridation at T greater than or equal to 600 degrees C yields rough and Ga-rich GaN layers. In both cases, it is shown that As segregates at the GaAs/GaN interface, indicating that GaAs plasma nitridation kinetics is limited by outdiffusion of As and/or AsN species.