The shape and nature of the chronoamperometric transients resulting from double-potential steps at rotating-disc electrodes, when the electrode reaction is of the ECE or DISP1 type, have been considered. It has been shown that such measurements can provide a clear discrimination between these two mechanistic pathways and this conclusion is verified in respect of the reduction of fluorescein in the pH range 9–10, which is shown to involve a DISP1 mechanism, in agreement with other observations.