Next, we present the results of combined CV and SXRD measurements performed on a Pt-3 electrode in pure and 0.1 M MeOH containing 0.5 M H 2 SO 4 . In this case, the potential was cycled between 0.05 V and 1.4 V, while the surface sensitive (1 0 2.35) reflection was monitored (Fig. ). The Experimental Section and Section S6 contain further information about the synchrotron measurements. For the sake of completeness, additional CVs and SXRD curves illustrating the effect of changing the UPLs from 1.05 to 1.40 V, are shown in Section S6, including also information on the noise in the CV data. CVs recorded in 0.5 M H 2 SO 4 on similar electrodes in the DEMS set-up are shown in (Figs. and). Despite the considerable noise, the CV (blue curve in Fig. ) resolves a slight increase in the current density in the positive-going scan above 0.8 V, which coincides with the onset of a steep decrease in the X-ray intensity. Previously, we had attributed a current increase (or peak) in this potential region to the desorption of bisulfate and the simultaneous adsorption of OH. Note that a possible exchange of bisulfate with OH in a 1:1 ratio would not show up in the CV since the net current density will be zero. Therefore, the actual onset for surface OH formation cannot be deduced directly from the CV and might even occur at lower potentials than the onset of the surface OH/O formation current. In general, the adsorption of OH/O will lead to a change in the interlayer spacing of the topmost Pt atoms, which is re-flected in a change in the X-ray intensity. In recent SXRD studies on the electro-oxidation of single-crystal Pt surfaces, a pronounced reversible decrease (positive-going scan) and increase (negative-going scan) of the SXRD intensity in the (0 1 1.5) and (1 1 1.5) 82 refelections was attributed to a reversible buckling of the Pt surface, involving a partial extraction of Pt surface atoms by reversible incorporation and removal of subsurface oxygen. On Pt(111), the onset for this process is at about 1.1 V. Earlier onsets for this process were reported for Pt(100) surfaces or carbon-supported Pt particles. Considering also the significant change in XRD intensity, we assume that the decrease observed here is primarily related to the reversible extraction of Pt atoms. From the present data we cannot distinguish whether this extraction occurs on the terraces or predominantly on defect sites (step edges and regions on the electrode that might not be as perfectly structured as those presented in the STM images in Fig. ). Changes in X-ray intensity due to desorption of bisulfate and adsorption of OH/O on the terrace sites are expected to be much smaller (see below). In the reverse, negative-going scan, the X-ray intensity starts to increase steeply at around 0.8 V, coinciding with the onset of a small reduction peak C2 in the CV. We associate this increase with the reversible removal of O atoms from subsurface sites and the de-buckling of the related Pt surface atoms, accompanied by reductive OH/O desorption and possibly bisulfate adsorption, which, however, does not fully compensate for the OH/O removal charge. After a complete potential cycle, the X-ray intensity reaches its original value. Further CVs recorded with increasing UPLs do not change significantly, suggesting that the surface did not irreversibly restructure substantially. Subtle changes in the CVs recorded for different UPLs, both in the combined SXRD-CV measurements as well as in CVs recorded in a dedicated electrochemical setup, are described and discussed in the SI. MOR measurements performed on these electrodes after the cyclic voltammetry measurements in the supporting electrolyte (red curves in Fig. ) resemble those recorded in the MOR on Pt electrodes. Adsorbed CO blocks the surface at low potentials, resulting in a negligible reaction current. The current starts to increase in the positive-going scan at around 0.6 V. Interestingly, while a detailed picture of the product composition is still missing, the formation of CO 2 close to the onset potential of the MOR is negligible. Hence, in this potential range, formaldehyde or formate / formic acid are the most likely products formed on Pt-3. After passing through a maximum at about 0.8 V, the current density decreases again and reaches negligible values at around 1.1 V. Formerly, it was suggested that this current density decrease is associated with OH/O adlayer formation on the surface, with adsorbed OH/O blocking the surface for the MOR. Also, in the negative-going scan, the electrode becomes only active for the MOR once the OH/O adlayer starts to be reductively removed at around 0.8 V (see below).