Electron energy loss spectra (ELS) have been obtained from polycrystalline Cr and Cr2O3 before and after surface reduction by 2 keV Ar+ bombardment. The primary electron energy used in the ELS measurements was systematically varied from 100 to 1150 eV in order to distinguish surface versus bulk loss processes. Two predominant loss features in the ELS spectra obtained from Cr metal at 9.0 and 23.0 eV are assigned to the surface and bulk plasmon excitations, respectively, and a number of other features arising from single electron transitions from both the bulk and surface Cr 3d bands to higher-lying states in the conduction band are also present. The ELS spectra obtained from Cr2O3 exhibit features that originate from both interband transitions and charge-transfer transitions between the Cr and O ions as well as the bulk plasmon at 24.4 eV. The ELS feature at 4.0 eV arises from a charge-transfer transition between the oxygen and chromium ions in the two surface layers beneath the chemisorbed oxygen layer, and the ELS feature at 9.8 eV arises from a similar transition involving the chemisorbed oxygen atoms. The intensity of the ELS peak at 9.8 eV decreases after Ar+ sputtering due to the removal of chemisorbed oxygen atoms. Sputtering also increases the number of Cr2+ states on the surface, which in turn increases the intensity of the 4.0 eV feature. Furthermore, the ELS spectra obtained from the sputtered Cr2O3 surface exhibit features characteristic of both Cr-0 and Cr2O3, indicating that Ar+ sputtering reduces Cr2O3. The fact that neither the surface- or the bulk-plasmon features of Cr-0 can be observed in the ELS spectra obtained from sputtered Cr2O3 while the loss features due to Cr-0 interband transitions are clearly present indicates that Cr-0 atoms form small clusters lacking a bulk metallic nature during Ar+ bombardment of Cr2O3. (c) 2005 Elsevier B.V. All rights reserved.