The interaction of SiO2surfaces with ultrathin layers of 4–16 A˚ of Cr evaporated in ultrahigh vacuum has been studied by x‐ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy. The surfaces were treated either by Ar sputtering, sputtering, and simultaneous annealing, or by annealing in O2. A room‐temperature reaction occurs on sputtered samples and produces a new XPS peak at a binding energy 4.9 eV lower than that of oxidized Si 2pand a shoulder in the O 1sline. These effects are less pronounced in sputtered‐annealed samples and insignificant in nonsputtered ones. Our results suggest the presence of silicon in a Cr‐rich environment which is at a maximum concentration away from the SiO2‐Cr interface, following a buffer region richer in oxygen.