X-ray photoelectron spectroscopy (XPS) depth-profiling analysis was performed to investigate the chemical composition of the film formed on Cu-Ni (90/10) alloy in seawater and sulphide-polluted seawater in the absence and presence of an inhibitor, 1,2,3-benzotraizole (BTAH). The chemical composition of the film is found to vary at different depths of the film. In seawater environment, the outermost layer is found to consist of CuO, Cu(OH)(2) and Cu2O. After sputtering for 12 min, the major constituent of the film is Cu2O. In seawater polluted with sulphide also the outermost layer is found to consist of CuO, Cu(OH)(2) and Cu2O without any Cu2S or CuS. After 4 min of sputtering only, the peaks due to Cu2S and CuS are detected and the major constituent of the film is still Cu2O. The sulphides of copper are absent after 8 min of sputtering. In the presence of BTAH, the XPS of the film showed peaks due to carbon and nitrogen up to 4 min of sputtering. This result reveals the presence of the [Cu(I)BTA](n) complex on the alloy surface in both seawater and sulphide-polluted seawater. The percentage of Cu2O is found to be much less than that found in the absence of inhibitor. In both the stated environments, in the absence of BTAH, the oxygen concentration is reduced to a small value only after 20 min of sputtering. Whereas, in the presence of BTAH, the concentration of oxygen is reduced to a very small value after sputtering for 12 min. These results infer the protective ability of the BTAH film on Cu-Ni (90/10) alloy in both seawater and sulphide- polluted seawater even after 30 days of immersion. (C) 2014 Elsevier B. V. All rights reserved.