The nitridation of the GaAs(001) surface using a radio frequency atomic nitrogen plasma source in a molecular beam epitaxy growth chamber has been studied. The resulting nitrogen induced GaAs(001) (3X3) reconstruction was investigated by in situ reflection high energy electron diffraction and x-ray photoemission spectroscopy (XPS). It was found that this reconstruction is only obtained in the temperature range 400-580 degrees C with a very low dose of atomic nitrogen. The nitrogen coverage corresponding to the (3X3) reconstruction was determined by quantitative XPS to be 0.30+/-0.09 ML, Below 400 degrees C an As-N species of disordered structure was found on the GaAs(001) surface. Subsequent annealing at about 500 degrees C produced the (3X3) reconstruction. Above 580 degrees C, nitridation lead to direct formation of beta-GaN islands. In addition, the (3X3) reconstruction was found to be unstable to both exposure to atomic hydrogen and annealing. The N desorption activation energy of the (3X3) was estimated to be 2.75+/-0.55 eV. A surface phase diagram of the (3X3) has thus been deduced.