We examine the effect of different anions in solutions containing benzotriazole (BTA) on the Cu removal rate during chemical mechanical planarization (CMP). In solutions containing both Cl- and BTA, the Cu removal rate is nearly a factor of twenty lower than in solutions containing either Cl- or BTA alone. As-grown BTA films from solutions containing different anions are characterized using atomic force microscopy, ellipsometry, Raman spectroscopy, mass spectrometry, and open-circuit-potential measurements. Films grown from halide-containing solutions are found to be considerably thicker than those grown from other anions. The difference in Cu removal rate correlates well with the different as-grown film thicknesses. (c) 2006 The Electrochemical Society.