The interaction of SiO2 surfaces with ultrathin layers of 4-16 Å 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 2p and a shoulder in the O 1s line. 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.