Surface-enhanced Raman scattering (SERS) of hydroxybenzoic acids has been studied on silver sols in H2O and D2O solutions. The adsorption behavior of 4-hydroxybenzoic acid (4HBA) is different from that of salicylic (2HBA) and 3-hydroxybenzoic (3HBA) acids. It was concluded that 4HBA is adsorbed on silver nanoparticles (Ag-n) as either oxidobenzoate (A(2-)) or hydroxybenzoate (A(-)), depending on the pH of the solution, given rise to a flat orientation. Both 2HBA and 3HBA acids are always adsorbed as hydroxybenzoates anions (A(-)) at pH >= 5 and link to the metal through the carboxylate group (Ag-n-A(-)), standing more or less perpendicular to the metal surface. In the case of these monoanions, the selective enhancement of the hands is due mainly to a resonant electron or charge transfer process (ET or CT) from the metallic nanoparticles to the adsorbates, yielding the transient formation of the respective radical dianions (Ag-n(+)-A(.2-)). It is found that the enhanced hands, and especially the mode 8a:nu(ring), are related to the difference between the equilibrium structures of the adsorbate in its ground (A(-)) and CT-excited (A(.2-)) states. In the SERS spectrum of 4HBA dianion, the contribution of CT mechanism is not observed. (c) 2008 Elsevier Inc. All rights reserved.