Sulphur dioxide is irreversibly adsorbed on a Pt(111) surface in sulphuric acid medium giving species that are very stable towards electrochemical oxidation. The electrochemical reduction of irreversibly adsorbed SO2 produces other strongly adsorbed species, one of which could be tentatively identified as atomic sulphur. SO2 adsorption followed by electrochemical removal of the resulting adspecies produces a structural modification of the Pt(111) surface. This structural modification is characterized by an increase in the number of (110) and (100) sites, by a diminution of the electric charge corresponding to the unusual adsorption states and by a shift of these states to more positive potentials.