Auger electron spectroscopy (AES) and reflection high-energy electron diffraction (RHEED) have been used to investigate the processes taking place during the initial stages of nitridation of GaAs(001)-2x4 surface by active nitrogen species. The results of analysis of the spectral shapes and the spectral positions of the Auger electron signals from Ga, As, and N, as well as their dependencies on the nitrogen exposure combined with RHEED results show that the processes taking place during nitridation greatly differ depending on the nitridation temperature. On the one hand, at low temperatures (T-s less than or equal to 200 degrees C) nitridation is hindered by kinetic restrictions on the atomic migration, leading to island growth with formation of the disordered GaAsN phase in the subsurface region, which cannot be completely re-crystallized even after prolonged annealing. On the other hand, at high temperatures (T-s greater than or equal to 500 degrees C) the process of nitridation takes place simultaneously with etching of the surface due to decomposition of the substrate, leading to growth of facets and resulting in a rough interface between the GaN and GaAs phases. However, for intermediate nitridation temperatures (300 degrees C less than or equal to T(s)less than or equal to 400 degrees C) it was found that the disordered GaAsN phase, formed at the initial stage of nitridation, partly crystallizes even without annealing. Moreover, AES results indicate that a monolayer of N atoms may well be formed on the substrate during the initial stage of nitridation, The postnitridation annealing of the samples nitrided at the intermediate temperatures results in the formation of a crystalline GaN layer. The line shape of the AES signals from this layer is identical to that for a thick GaN reference sample.