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Passivation of semi-insulating indium phosphide surfaces by various types of oxides is investigated with infrared absorption spectroscopy (IRAS) and X-ray photoelectron spectroscopy (XPS). The difficulty in studying oxide-related modes in the 800–1300 cm−1 range is due to the bulk phonon absorption of the InP substrate and has stimulated the optimization of single-pass “external” transmission as well as multiple internal reflection geometries for ex situ and in situ studies, respectively. The nature of oxides prepared commercially for epitaxy (epi-ready) and oxides prepared by wet chemistry is studied as well as the wet chemical etching of such oxides, including the reactivity of InP in various solutions. Combining XPS and polarization-dependent IRAS, we find that epi-ready oxides tend to be porous, and plasma-grown oxides denser. [Copyright &y& Elsevier]