The adsorption, from aqueous solution, of (S)-aspartic acid onto polycrystalline Ni was investigated as a function of pH via X-ray photoelectron spectroscopy in order to investigate why the hydrogenation of beta-ketoesters over aspartic acid-modified Ni catalysts exhibits a very sharp maximum in enantioselectivity at modification pH 5. Following similar modification and washing procedures to those used in the real catalyst preparation, it was found that, under the optimum preparation conditions, the coverage of aspartic acid on the Ni surface drops below detectable levels. The implications for understanding the mechanism of the enantioselective hydrogenation reaction are discussed. (C) 2010 Elsevier Inc. All rights reserved.