A study is presented on the cathodic reduction in ClO3- ions in acidic and circumneutral solutions. The system's thermodynamics (i.e., the corresponding Pourbaix diagram) is an aid in predicting the appropriate conditions for the different processes to occur and in interpreting the results. Linear sweep and cyclic voltammetries on a Pt electrode, together with a spectroelectrochemical arrangement that allows for real-time monitoring of these processes by means of both a UV/visible spectrophotometer and a potentiostat, yield evidence of the ClO3- to ClO2- reduction in circumneutral media and of a multiplicative effect in the production of ClO2 in highly acidic sulfate media due to a chemical/electrochemical/chemical pathway. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3121588] All rights reserved.