The observed loss of activity at constant conditions during the selective oxidation of ethanol with oxygen in a continuous stirred-tank reactor with carbon-supported platinum can be described by a model considering reversible transformations between three oxidizing species on the catalyst. One of these species is much more reactive toward ethanol and can be considered as a reaction intermediate in the selective oxidation of the latter. The model is also able to simulate the relaxation of the catalyst potential when the reaction is performed with a platinum foil in an electrochemical cell. The loss of activity as well as the relaxation of the catalyst potential can be attributed to changes in the degree of coverage by the two less reactive forms of oxygen. The latter should be considered as an extrinsic relaxation in contrast to the establishment of the steady-state degree of coverage by the reaction intermediates in the selective oxidation of ethanol, i.e., the intrinsic relaxation.