DSA® metal oxide electrodes such as the RuO2/IrO2/TiO2 mixed system are widely studied for their excellent electrocatalytic activity. In order to understand their catalytic properties, the comprehension of the surface chemistry involved during electrochemical treatments is crucial. With this aim, RuO2/IrO2/TiO2 mixed-oxide electrodes having various noble metal contents were studied by means of secondary ion mass spectrometry (SIMS). In particular, cathodic and anodic polarization and O2 evolution reactions were carried out to test the electrode behaviour and SIMS analyses were performed after all these treatments. In this way, surface changes induced by electrochemical treatments and depending on electrode composition were widely investigated by SIMS, revealing, for example, the presence of hydration or preferential dissolution phenomena induced by electrochemical processing. Copyright © 2004 John Wiley & Sons, Ltd.