The methanol crossover properties of various membrane electrolytes are shown to be measurable by means of a modified differential electrochemical mass spectrometry (DEMS) [Bittins-Cattaneo, B., Cattaneo, E., Konigshoven, P. and Vielstich, W., in Electroanalytical Chemistry, Vol. 17, ed. A. J. Bard, Marcel Dekker, New York, 1991, p. 181; Welter, O., Giordano, M. C., Heitbaum, J., Vielstich, W., O＇Grady, W. E., Ross, P. N. and Will, F. G. (Eds.), Proc. Symp, Electrocataly sis, Minneapolis 1981. PV 82-2, The Electrochemical Society, Pennington, NJ, 1982, p. 235.] technique using experimental electrodes and gas-diffusion electrodes. Electrochemically synthesized crosslinked substituted polyoxyphenylene (POP) films on gold and platinized carbon supports were the targets of electrode modifications and a screening study of these polymers and commercially available ion exchange membranes was carried out. POP-like polymers are expected to be an essential ingredient in future crossover minimization strategies.