A roughened silver electrode modified with silver nanoparticles is used as a substrate, on which high-quality surface-enhanced Raman spectroscopy (SERS) of isonicotinic acid (INA) are obtained, indicating that the silver-modified silver electrode is a highly SERS-active substrate. It is difficult to separate the contributions of the electromagnetic and chemical mechanisms to the great enhancement of the Raman signal. The shift by 5-20 cm(-1) of the surface-enhanced Raman spectroscopy (SERS) bands and the change in their relative intensity compared with the ordinary Raman spectrum indicate the chemisorption of the sample molecules on the silver-modified silver surface. Furthermore. the silver nanoparticles modified on the roughened silver surface play the important role of magnifying the surface local electric field near the silver surface through resonant surface plasmon excitation. From the rich information on the silver-modified silver electrode obtained from high-quality potential-dependent SERS, we may deduce the adsorption behavior of INA and the probable SERS mechanism in the process. The probable reasons are given. (c) 2005 Elsevier Inc. All rights reserved.