We have investigated the nitridation of GaAs(110) and (100) by photodissociation of the adsorbed hydrazoic acid, HN3, at 308 nm and surface temperature of 120 K with HREELS, XPS and TDS analyses. At low coverages, the photodissociation of HN3(a) produced mainly NHx(a), whereas at high coverages, the photodissociation of multilayer HN3 yielded NHx(a), N2(a) and N3(a) as revealed by XPS and HREELS. The TDS of the irradiated multilayer sample gave an early N2 peak at 150 K, in addition to the one which tracked that of the parent HN3 above 160 K. The high temperature TDS contained a single AsN peak at 720 K before the decomposition of the substrate above 800 K, which yielded AsN, GaN and fragments of GaAs. The presence of these nitrides on the surface is consistent with the results of HREELS and XPS measurements. For GaAs(110), the decomposition products, NHx species formed bonds with surface As and Ga atoms as indicated by the 90 meV peak in HREELS and by the appearance of new peaks in As 2p 3 2 and Ga 2p 3 2 XPS. Annealing of the laser-irradiated sample at 450 K caused the partial desorption of the adsorbates and the breaking of the GaAs surface bonds as well as the NH bond. Meanwhile, the As 2p 3 2 XPS peaks at 1323.7 and 1325.1 eV shifted to 1324.5 and 1325.9 eV, respectively. The former peak is attributable to the AsN species with the N atom back-bonded to the Ga atom, while the latter peak could be attributed to the As atoms bonded to two NHx species. The results for the GaAs(100) surface were similar but the relative concentration of As nitride was generally lower, probably because the top layer atoms are Ga instead of As. The He(I) UP spectrum showed a significant surface states reduction after a GaAs(100)c(8 × 2) surface was exposed to HN3 and 308 nm photon beams and then annealed at Ts > 750 K.