The cathodic potential-decay process on platinum electrodes in oleums has been investigated by means of a rotating disk. The non-stady parameters are compared with those already reported under steady-state conditions. Both results are discussed in terms of a general reaction mechanism comprising various consecutive as well as alternative steps, considering that the main cathodic product is sulphur dioxide. The SO3-containing species acts as a depolarizer by reacting with hydrogen atoms adsorbed on the electrode surface.