The electrochemical oxidation of ethylene on platinum electrodes in 1N sulfuric acid at 80 °C was studied by the potential sweep method over a wide range of sweep rates . The results were compared with those obtained previously under steady‐state conditions. Over three decades of sweep rate an apparent Tafel slope of approximately was observed. This was shown to be consistent with the behavior for a totally irreversible diffusion‐controlled reaction. In the same region the peak potential was found to shift 65 mv/decade increase in sweep rate and the peak current was proportional to the square root of the sweep rate with a proportionality constant in agreement with the calculated value.It is shown that the application of the potential sweep may change the mechanism of the process taking place (in the present case the change is from activation control in steady state to diffusion control during potential sweep). During potential sweep a totally irreversible, diffusion‐controlled reaction will give rise to a linear Tafel plot with a slope about 35% higher than the mechanistically significant slope of 2.3 RT/αF. Further, an apparent passivation region can be observed, due to depletion of the solution near the electrolyte from reactant molecules and quite independent of the nature of the electrode surface. During potential sweep measurements the partial coverages θ by various species on the surface cannot reach their steady‐state value. The kinetic parameters derived from such measurement, therefore, cannot be used to represent the results under steady‐state conditions, unless the sweep rate is made sufficiently slow to correspond in practice to steady state.