High-resolution electron energy-loss measurements performed in situ on GaAS(001)-c(4 x 4) surfaces grown by molecular beam epitaxy reveal a number of intrinsic features so far unobserved. The one-to-one correspondence found between experimental and calculated electronic transitions provides for the first time a satisfactory understanding of the electronic properties of this prototype surface. The optical surface anisotropy originates entirely from very few atomic layers beneath the surface. This gives to the energy-loss anisotropy data two orders of magnitude higher surface sensitivity than that of reflectance difference spectroscopies. (C) 2002 Elsevier Science B.V. All rights reserved.