The surface of high-density sintered MgB2 was characterized by Auger electron spectroscopy (AES) in combination with sputter depth profiling. The compositional changes of the MgB2 surface were investigated in real time during the course of Ar+ ion sputtering. The spatially-resolved correlation of surface composition with surface morphology was revealed by imaging the distribution of elements on the MgB2 surface. The as-polished MgB2 surface was found to be Mg-rich and oxidized, especially near the grain boundaries. Therefore, careful protection of MgB2 from ambient atmosphere is required for its practical applications. The sputtered MgB2 subsurface region was found to be Mg deficient, especially on the top of crystallites. The detailed compositional analysis of the MgB2 surface in this paper may help in explaining the reported varied scanning tunneling spectroscopy (STS) results. Our AES results presented here show that an MgB2 superconductor surface being treated by common Ar+ ion sputtering method may not be suitable for surface-sensitive measurements. (C) 2002 Elsevier Science B.V. All rights reserved.