A surface characterization study using x-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) has been performed on polished, solvent-cleaned, undoped InP(111) substrates before and after room-temperature exposure to the flux produced by a novel atomic hydrogen source based on electron-stimulated desorption of hyperthermal (1-3 eV) hydrogen atoms from a Cu-alloy membrane. The native oxide layer on the solvent-cleaned InP(111) substrate is nonhomogeneous and contains primarily C, O, and In, and very little P. Indium is present in the near-surface region as InPO4, In(PO3)(3), InPO3, InP, and a relatively small amount of In2O3 in the subsurface region. Phosphorus is present as InPO4, In(PO3)(3), InPO3, InP, P2O5, and elemental P. For In and P, InP is the predominant form. Before H-atom exposure the C is present as hydrocarbons, alcohols, and carbide with hydrocarbons as the predominant chemical state. During room-temperature exposure to the hyperthermal H-atom flux for 90 min, removal of oxygen and carbon contamination occurs with the O content decreasing by about 28% and the C content decreasing by 93% according to the XPS data. The In:P ratio is initially 2.28 indicative of an In-rich near-surface region. After a 15 min and then 90 min H-atom exposure, the In:P ratio decreases to 1.30 and 1.11, respectively. Complex chemical changes occur during the H-atom exposures. The P2O5 is eliminated, the In phosphates are converted to biphosphates and hydroxide and all forms of C are removed. Some of the phosphates and In2O3 are decomposed by the H-atom flux, but these chemical reactions occur slowly at room temperature. According to ISS data, the H-atom flux is very effective in removing contamination at the outermost atomic layer which is crucial for epitaxial growth of device-quality thin films.