The employment of novel SERS-active colloidal substrates, constituted by Ag-doped silica nanoparticles, allows monitoring the occurrence of the central line at similar to 1025 cm(-1) between the two intense SERS bands of pyridine at 1008 and 1036 cm(-1) during the photoreduction of silver ions with visible laser irradiation. The origin of this band, previously observed in pure Ag hydrosols and in electrochemical cell, is attributed to positively-charged silver clusters bound to the pyridine molecules. These species, stabilised by interaction with the silica surface, are disrupted by coadsorption of chloride anions onto the silver substrate, with the consequent disappearing of the central line. (c) 2005 Elsevier B.V. All rights reserved.