Adsorption of the corrosion inhibitor benzotriazole (BTA) from solution has been studied on well-characterized electrochemically prepared Cu surfaces. In situ ellipsometry and ex situ x-ray photoelectron spectroscopy (XPS) have been used to probe both the nature of the Cu surfaces prior to BTA adsorption and to determine the stoichiometry, thickness, and chemical composition of the Cu-BTA overlayer films. The films grown on Cu20 and on Cu° under oxidizing conditions are generally 5–40 A thick and best described as Cu+1BTA. Uptake of BTA is also observed on CuO and reduced Cu° surfaces where the adsorbed film is limited at close to a monolayer. Therefore, Cu20 does not appear to be a prerequisite for uptake of BTA. The thickness of the inhibitor overlayer was calculated from the N Is/ Cu 3p core level XPS data and was in good agreement with independent ellipsometric measurements on similar samples. Roughening of the Cu surface is observed for samples treated at low pH. Differing amounts of oxygen are found in the Cu+1BTA films depending upon the preparation environment. © 1990, American Vacuum Society. All rights reserved.