In contrast to Fumasep®, Selemion™ was found to lose only 3 ± 3% of the original thickness, well within the margin of error for the thickness measurement. The values of tmemb and σmemb for Selemion® are compared with Fumasep® and summarised in Table . The Selemion™ membrane not only maintained conductivity much better than Fumasep®, (25 ± 6% compared with 82.8 ± 1.9% by 12 h), but, as with the HFR, the conductivity loss only occurred during exposure to synchrotron radiation while the σmemb increased during the current holds, indicative of further membrane conditioning. The minimal change to the thickness and conductivity of the Selemion™ membrane reflects the stability of its polystyrene-based, aryl ether free, backbone in the high pH environment and while exposed to synchrotron radiation.