Polypropylene membranes were modified by the plasma etching of SO2, SO2-O-2, or SO2-H2O, followed by the plasma polymerization coating of SO2/acetylene. The conditions for SO2 plasma etching were optimized by the measurement of the ion-exchange capacity (IEC) as a function of the plasma-etching power (10-30 W), gas pressure (40-60 mTorr), and treatment time (15-120 s). For the plasma etching of SO2-O-2 and SO2-H2O, only the pressure ratio (SO2/O-2 and SO2/H2O) was optimized under the optimized conditions determined from SO2 plasma etching. Plasma etching was then combined with the plasma polymerization coating of SO2/acetylene, for which the conditions were again optimized by the measurement of the IEC as a function of the plasma power (10-40 W), chamber pressure (50-200 mTorr), SO2/acetylene ratio (15/135-60/90), and treatment time (0-10 min). Next, the electrical resistance and water uptake were evaluated. The modified membranes were also analyzed with scanning electron microscopy, whereas plasma polymer coatings were characterized with Fourier transform infrared/attenuated total reflection. (c) 2006 Wiley Periodicals, Inc.