The effect of active nitrogen, generated by a radio frequency plasma source, on clean GaAs (001) surfaces has been examined using x-ray photoemission spectroscopy (XPS) and reflection high energy electron diffraction. The "nitridation" of the surface was performed under fixed plasma conditions, compatible with the molecular beam epitaxial growth of GaN, and as a function of both temperature (in the range similar to 300-600 degrees C) and time (up to 15 min). At low temperatures the nitridation proceeds very slowly and was characterized, in its initial stages, by the transformation of the (2 X 4) reconstructed GaAs surface to a high intensity "amorphous" haze, presumed to be related to the As released in the anion exchange reaction but not evaporated from the surface. At high temperatures the nitridation is much more aggressive readily forming thicker GaN films of a polycrystalline nature. Curve fitting of the XPS spectra, to reveal the nature of the reaction products indicated the probable formation of As-N species in addition to GaN.