The adsorption behavior of uracil. an excellent ligand in cyanide-free silver plating on silver electrodes was studied with potential-dependent surface-enhanced Raman spectroscopy (SERS). Uracil was adsorbed at the silver surface with N3-deprotonated tautomer at pH 10.2, while with N1- and N3-deprotonated tautomers at pH 13.6. Uracil and additive polyethyleneimine (PEI) exhibit a co-adsorption behavior that holds the same tendency as their adsorption on the silver surfaces alone. The effect of PEI on silver plating process is proposed from SERS and electrochemical analysis. PEI, acted as an efficient suppressor and grain refiner in this silver plating bath, was further confirmed by the AFM micrographs. (C) 2009 Elsevier B.V. All rights reserved. ; National Science Foundation of China [90407019, 20775015, 20735002]; State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen University