A comparison of the stability test data from this operando synchrotron experiment and lab-based testing of nominally identical CCMs prepared with Fumasep® FAA-3-50 membranes is presented in Figure . While Figure shows that the Ecell,iR corr. for both tests was of comparable magnitude up to 16 h, it increased dramatically after this point, indicative of catalyst loss. The overall Ecell,iR corr. increase during the stability test progressed linearly at 23 mV h⁻¹ after the first 4 h compared with 6 mV h -1 measured for the lab-based test. This high rate of Ecell,iR corr. increase, unique to the operando test, was likely caused by X-ray related breakdown of the ionomer in the catalyst layer as similarly found by Roth et al. for PEM fuel cell ionomers upon exposure to synchrotron radiation. Figure shows a dramatic rise in the HFR throughout the operando synchrotron test. The HFR increased most when the cell was exposed to synchrotron radiation while μ-CT was performed, as seen by the step changes in the HFR going from the before scan (solid squares) to after scan (open squares) measurements. During each current hold, between μ-CT scans, the HFR did not change significantly. In contrast, the lab-based test which was not exposed to synchrotron radiation showed a comparatively negligible increase in the HFR throughout the stability test (Figure ). These findings clearly demonstrate the significant effect of synchrotron radiation on the ohmic resistance measured through the HFR. Although X-ray imaging is typically considered non-destructive, polymer electrolyte membranes and ionomers are highly susceptible to radiation damage, particularly from high flux synchrotron sources. The extent of this damage varies depending on the polymer type and the dose of exposure. Therefore where unavoidable, accounting for the effects of radiation damage during operando tests is crucial for distinguishing between electrochemical and beam-related degradation. While the effect of synchrotron radiation on the HFR was clear, the effect on membrane thickness required further examination. A reference experiment was designed to examine the effect of synchrotron radiation on the electrolyser independent of electrochemically driven degradation. In this experiment, the cell was not subjected to high current density, only 0.05 A cm -2 was applied so that EIS could be performed, to avoid degradation mechanisms stemming from the electrochemical operation of the CCM. An equivalent series of μ-CT scans each followed by EIS was completed, for further details on the methodology, see Section S1.3 in the SI. The change in the HFR and tmemb were measured against the X-ray dose from each μ-CT scan.