High resolution electron energy loss spectroscopy, dielectric theory simulations, and charge profile calculations have been used to study the accumulation layer and surface plasmon excitations at the In-terminated(001)-(4×1)and(111)A-(2×2)surfaces of InAs. For the (001) surface, the surface state density is4.0±2.0×1011 cm−2,while for the(111)Asurface it is7.5±2.0×1011 cm−2,these values being independent of the surface preparation procedure, bulk doping level, and substrate temperature. Changes of the bulk Fermi level with temperature and bulk doping level do, however, alter the position of the surface Fermi level. Ion bombardment and annealing of the surface affect the accumulation layer only through changes in the effective bulk doping level and the bulk momentum scattering rate, with no discernible changes in the surface charge density. ©1997 American Institute of Physics.