Nitridation of GaAs(1 0 0) is performed using a double differential pumping RF cell. X-ray photoelectron spectroscopy (XPS) is used to monitor surface composition changes that result from ionic etching by Ar+ bombardment and nitridation of this substrate. We have studied the influence of temperature and pressure of nitrogen. The process of nitridation leads to the formation of a nitride layer which seems to be a ternary compound of gallium, arsenic and nitrogen. To improve the composition of the surface layer and to obtain pure GaN thin films on the surface by stopping arsenic migration, we have performed another kind of nitridation process. It consists, firstly, of a deposition of metallic gallium on the substrate surface. The system Ga/GaAs is then nitrided and the XPS measurements indicate the formation of a GaN layer on the surface. (C) 2003 Elsevier Science B.V. All rights reserved.