The removal of the native oxide overlayers on the surfaces of III-V semiconductors, although widely investigated, requires further examination. Specifically, the main emphasis of research has been on the oxide layer characterization and, hence, on the development of cleaning procedures. This article attempts to study the H-atom plasma reduction of the native oxide on indium phosphide (InP) substrates, using a process parametric investigation and x-ray photoelectron spectroscopy (XPS) analysis. Oxide-free and stoichiometric InP surfaces are prepared by operating the plasma cleaning at the surface temperature of 270 degrees C, as a thermally activated process has been found. For the untreated substrate, the nature of the InP oxide changes, along the thickness, from indium phosphates (InPOx) to an indium oxide (In2O3) rich sublayer. High temperature plasma treatment is able to completely reduce the In2O3 oxide, unlike the wet-etching procedure. In addition, improved stability of the surface against reoxidation has been confirmed for plasma treated substrates.