The photoelectrochemical modification of p-InP( I I I) Surfaces in HCl and H2SO4 is investigated. In 0.5 M HCl a photoreduction/oxidation procedure via cyclic voltammetry leads to a solar energy conversion efficiency of the p-InP/vanadium(2+.3+) -4 M HCl/C photoelectrochemical solar cell (PECS) of eta = 11.6% in natural sunlight. The optimisation procedures are followed by Surface analysis methods performed ex-situ at the modified InP electrodes after cathodic and after anodic polarisation in the Supporting acidic electrolytes and in the redox Solution. X-ray photoelectron spectroscopy (XPS). ultraviolet photoelectron spectroscopy (UPS) and low energy electron diffraction (LEED) are used. At cathodic polarisation in H2SO4. XPS shows formation of indium metal and In-x(SO4), compounds, whereas in HCl a stable interfacial film consisting of indium monochloride to a smaller extent of indium oxides is formed. This film is optimised by cycling in HCl having a thickness of 10 +/- 3 Angstrom. Cyclic polarisation in the vanadium redox electrolyte changes the composition and thickness. Whereas in HCl the optimised interphase consists of 0.7 parts InCl and 0.3 parts In(PO3)(3), phosphates area bsent after optimisation in V2+/3+ and more indium Oxide is found. The film thickness is reduced to 6 +/- 2 A for the InCl component and about 0.6 monolayers of indium oxide are found. UPS measurements are mainly used to derive an energy band diagram of the semiconductor/film/electrolyte junction. The results show in that the stabilisation of the film in chloride containing electrolytes can be explained by the relative conduction band positions of p-InP with respect to cathodic decomposition energies. Since the electron affinity increases with surface phase formation in HCl, whereas it stays low in sulphuric acid. the ongoing corrosion in the latter solution can be understood, the LEED data show that the surface phase is ordered with lattice constants close to InP, indicating the formation of a hitherto unknown In-O Cl phase. According to the results we propose reaction mechanisms for dissolution and stabilisation and present schematic energy band diagrams which explain the corrosion processes in H2SO4 and the passivation phenomenon in HCl. The influence of the properties of the interfacial film oil the functioning of the solar cell is outlined.