The influence of bromine on the surface of a superconducting oxide (Y1Ba2Cu3O7-x, or YBCO) has been studied using X-ray photo-electron spectroscopy. It was seen that exposing the oxide to pure, dry Br-2 vapor changes both the composition and chemistry of the near-surface region. The chemical coordination of O2- and Y3+ ions is completely altered in the detected region (1-3 nm). The reaction products are expected to be metal-Br-O compounds, possibly combination of bromates and bromides. This is accompanied by severe redistribution of the cations: the surface is enriched in Y3+ and depleted of Ba2+. When etched in Br/ethanol solution and rinsed in ethanol, the reaction products are mostly washed off leaving a trace of Y-Br containing film. The mechanisms of Br-induced segregation in this compound should be carefully investigated in view of the fact that Br is known to influence this superconductor in many beneficial ways. (C) 1998 Elsevier Science B.V.