The nitridation of ultra-thin Ti films on Si(1 0 0) have been studied using X-ray photoelectron spectroscopy (XPS) in a temperature range of 120-1000 K. Upon ammonia exposure to the multilayer Ti thin films at 120 K, three N is peaks at 397.8-398.1. 400.5-400.8 and 402.2-402.6 eV were observed, attributable to NHx (x = 1 or 2). molecular NH3 and NH4sigma+, respectively. Annealing of the NH3 saturated Ti/Si(1 0 0) surfaces results in the conversion of the NHx species, This species undergo two different pathways between 300 and 800 K, i.e. further dissociation to N(a) and H(a), and recombing with H(a) to form NH3(g). The atomic N reacts with Ti to yield a stable TiN film that retards significantly the interdiffusion at the Ti/Si interface.