The interaction between vacuum-deposited Cu and a carboxylic acid-terminated self-assembled monolayer (SAM) on Au(111) has been investigated using high-resolution X-ray photoelectron spectroscopy (XPS). Upon adsorption of 3-mercaptopropionic acid from the gas phase on An(111), C 1s, O 1s, and S 2p line shapes and binding energies are consistent with the formation of a thiolate surface intermediate with an intact carboxylic acid function. Deposition of increasing amounts of Cu results in preferential modification of the hydroxyl group O 1s peak indicating unidentate complexation. Consistent with selective bonding, Cu induces changes in the carboxylic acid C 1s region with no evidence of reaction with the methylene or thiolate units. The Cu 2p core level is comparable with bulk, suggesting cluster growth and weak Cu-SAM interactions. The reactivity is nevertheless sufficient to prevent metal penetration to the An substrate surface. (C) 2004 Elsevier B.V. All rights reserved.