Chemical treatment by hydrazine-sulfide solutions, known to produce surface nitridation of GaAs(100), was applied to GaAs(111) A and B surfaces. The chemistries of these treatments for the Ga-terminated A surface and the As-terminated B one were investigated using synchrotron radiation photoemission and Auger electron spectroscopies. For the B surface, such treatment was found to produce an effective surface nitridation, via substitution of surface arsenic with nitrogen atoms from hydrazine molecules. The process automatically stops after formation of one monolayer. In contrast, the A surface is covered by sulfur bonded to underlying gallium. This extreme dependence on surface polarity is explained by competitive adsorption processes of HS- and OH- anions and of hydrazine molecules, on Ga-adsorption sites, which have distinct configurations on the A and B surfaces. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3529936] All rights reserved.