Plasma-induced surface damage of a III-V compound semiconductor, a problem associated with many device fabrication processes, is clarified with careful measurements of surface defect density induced by low energy ion bombardment of InP. In the study, n- and p-InP (110) surfaces were prepared by cleavage of InP in ultrahigh vacuum, and then bombarded as a function of ion type (He+ and Ar+), energy (5-100 eV), and fluence (10(12)-10(17) ions/cm(2)). The dynamic process of surface Fermi level shifting induced by such bombardment was determined by in situ high-resolution x-ray photoelectron spectroscopy, and the data were then converted to information on surface defect formation. It was found that both He+ and Ar+ bombardment with the above conditions moved the Fermi levels of both n- and p-InP (110) surfaces towards 0.95 eV above the valence band maximum of InP. As expected, for the same bombardment energy, Ar+ caused more damage than He+, and for the same ion type, the bombardment induced a surface defect density. increasing with both ion energy and fluence. It was also found that the threshold condition for defect formation was a combined function of the impact energy of the incoming ion and the energy released during its neutralization.