The evolution of the X-ray intensity at the (1 0 2.35) position during the MOR (red curve) is very similar to that obtained in the pure supporting electrolyte, except that the onset of the sharp decrease in the positive-going scan shifts by 300 mV to more positive potentials, from 0.8 V to 1.1 V. As mentioned above, the reversible insertion of adsorbed O atoms together with the partial extraction of the associated Pt surface atoms was suggested to be the dominant process in this potential region in the pure supporting electrolyte, which accounts for this change. Still, there is a slow decay in X-ray intensity in the potential range between the MOR peak maximum at about 0.8 V and the end of this peak at about 1.1 V, which is possibly related to the adsorption of OH/O on the surface, inhibiting the MOR. The up-shift to 1.1 V of the steep decrease of the X-ray intensity indicates that the associated process is mostly inhibited in this potential range during the MOR, as compared to measurements in the pure supporting electrolyte on Pt-3. Most likely, this up-shift is caused by a reaction of OH/O (possibly at defect sites as described above) with methanol or with reaction intermediates formed during the MOR, which lowers the steady-state OH/O coverage at a given potential and thus shifts the coverage-potential curve to higher potential, in agreement with the up-shift of the onset of the steep decline of the X-ray intensity observed experimentally. Surface-enhanced infrared reflective adsorption spectroscopy (SEIRAS) measurements on pure polycrystalline Pt electrodes indicated that the adsorbed CO blocking the surface before the MOR onset is entirely removed at the MOR peak maximum. Hence, also adsorbed CO could block the OH/O adsorption and consequently also O insertion / Pt extraction in the potential range up to the X-ray intensity decrease. Furthermore, with the onset of the MOR, adsorbed formate (HCOO ad ) was found to form on the surface, with the SEIRAS intensity following the MOR current density in both scan directions, which can contribute to the slow decay in X-ray intensity in this potential region.