An anodic photo-galvanostatic treatment at low current density (1 mu A.cm(- 2)) is carried out on n-InP semiconductor in liquid ammonia (223 K). The gradual chemical evolution of the surface is studied as a function of the anodic charge. Proof and reproducibility of the chemical transformation of the surface are clearly evidenced by X-ray photoelectron spectroscopy (XPS) analyses. Like by cyclic voltammetry, the perfect coverage of the InP surface by a thin phosphazene like film is also revealed by XPS data. However, a low anodic charge (approximate to 0.5 mC.cm(-2)) is required by photo-galvanostatic treatment while a higher anodic charge (approximate to 7 mC.cm(-2)) is involved by cyclic voltammetry. The excess of charge could be related to ammonia oxidation during the formation of the passivating film. This result proves the electrochemical oxidation of the solvent as a determinant step of the mechanism film formation. (c) 2012 Elsevier B.V. All rights reserved.