Interestingly, the HFR was found to decrease during the 2 h, 1 A cm -2 holds, behaviour only seen for the Fumasep® based cell during the first two current holds when Ecell,iR corr. was below 2 V. This observation propagated through to calculated conductivity values shown in Figure , where the σmemb increased for Selemion™ during the current holds while it decreased during exposure to synchrotron radiation. This observation could be evidence of continued conditioning during the current cycling where more of the chloride counterion in the membrane is replaced by hydroxide. For Fumasep® however, the combined effects of the HFR increase and membrane thinning resulted in an almost constant loss of σmemb.