Benzotriazole (BTAH) is well known as an effective corrosion inhibitor for Cu because of its ability to make a coordination polymer film on the surface that provides a barrier to Cu oxidation. BTA− film formation was investigated on single-crystal and polycrystalline Cu surfaces with shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) using silica-encapsulated Au nanoparticles. Potential-dependent spectra display reversible film formation on polycrystalline Cu and irreversible film formation on single-crystal Cu. Grain boundaries leading to smaller BTA−-Cu oligomers are proposed to be the reason for cathodic degradation of the BTA− polymeric films on polycrystalline Cu. Copyright © 2011 John Wiley & Sons, Ltd.