The reaction pathway of sulfur dioxide electro-oxidation on platinum and gold substrates is investigated here using a combination of static and rotating electrode voltammetric methods. A gold electrocatalyst does not need to be sulfur modified in order to display high activity, meaning the substrate is inherently more active than platinum. Analysis of diffusion behavior and reaction order on these substrates suggests that both kinetic and diffusion limitations control oxidation on each electrode. However, it is proposed that a similar oxidation pathway, involving adsorption and subsequent oxidation of sulfur dioxide on the electrode surface, is followed for an activated electrode. Mass changes were monitored at the electrode surface using an electrochemical quartz crystal microbalance, and both in-and ex-situ observations support this pathway proposal. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.060209jes] All rights reserved.