The electrochemical reactivity of benzoic acid on porous platinum and palladium electrodes has been studied by cyclic voltammetry and differential electrochemical mass spectrometry (DEMS) in aqueous 0.1 M HClO4. The objective of this work is to investigate the adsorption processes and the reactivity of this compound on different noble metals, in order to compare these results with those obtained for related aromatic compounds. On-line mass spectroscopy analysis of volatile products reveals that the adsorption of benzoic acid is irreversible at platinum while it is mainly reversible on palladium. Accordingly, different catalytic activity of platinum and palladium is demonstrated during the electrooxidation process. The anodic stripping of the adsorbates formed from benzoic acid yields CO2 as the sole oxidation product of platinum. Cathodic stripping of the residues results in the partial desorption of the organic adlayer from the electrode. At palladium, only small amounts of irreversibly adsorbed species, which also oxidise to CO2 are detected. The effect of the nature of the electrode on the electrochemical behaviour of these substances is also analysed. (C) 2000 Elsevier Science B.V. All rights reserved.