The electrochemical oxidation of formic acid under potentiostatic conditions was found to exhibit transient oscillations on all three low-index plane Pt surfaces in an unstirred electrolyte. Depending on structure, substantial differences occurred concerning induction time, oscillation period, and shape as well as number of oscillations. Stirring could stop the oscillations, but also restart them when they had faded spontaneously. It is shown that a potentiostatic model, based on the branched oxidation path of formic acid and taking the pH in the double layer into account, can qualitatively explain the observed phenomena.