The chemical bonds established on GaAs(001) by wet nitridation in hydrazine solutions containing 0.01 M of sodium sulfide are investigated using in-situ IR absorption spectroscopy. Hydrazine distillation is found to be necessary in order to avoid numerous carbon-related features in the spectra. The absorption spectrum consists of positive lines due to adsorbed hydrazine molecules and of negative lines due to water being removed from the surface. Well-defined structures near 960 cm(-1) and 1020 cm(-1) as well as a broader line near 1008 cm(-1), not present in the absorption spectrum of the solution, also appear. Investigation of isotopic effects, of the effect of plasma nitridation and of the kinetics of the various lines suggests that hydrazine bonding first occurs to surface Ga atoms in a singly-bonded (Ga-NH-NH2) and doubly-bonded (Ga)(Ga) > N - NH2 end-on configuration. At a later stage, dissociative interaction between chemisorbed hydrazine molecules forms the (Ga)(Ga) > NH configuration. (C) 2013 The Electrochemical Society. [DOI: 10.1149/2.063304jes] All rights reserved.