We prepared and characterized a series of semi-IPN (semi-interpenetrating polymer network) membranes based on SPISX-EP (sulfonated polyimide-siloxane and epoxy) polymers and compared their properties with those of a pure SPISX membrane and a commercially available proton exchange membrane. Overall, the SPISX5-EP (with 5 wt% PDMS (alpha,omega-diaminopropyl polydimethylsiloxane) in SPISX) membranes can be exhibited desirable mechanical properties and thermal stabilities, with proton conductivities superior to those of Nafion (R) 117 at 80 degrees C. The dimensional changes of the membranes and degrees of methanol transport decreased with increasing epoxy content; here, the effect of crosslinking had a greater effect than did the increased number of ionic exchange sites. The proton conductivities and methanol permeabilities of the membranes ranged from 10(-3) to 10(-2) S/cm and from 10(-9) to 10(-7) cm(2)/s, respectively, in the temperature range 25-90 degrees C. Transmission electron microscopy images indicated that the presence of large, well-connected hydrophilic domains was responsible for the large hydrolytic stability of the SPISX5-EP membranes; infrared spectroscopy confirmed these results. SPISX5-EP membranes featuring epoxy compositions of 30 and 40% exhibited greater hydrolytic stability relative to the corresponding SPISX membranes, suggesting their potential application as proton-conducting membranes in fuel cells. (C) 2013 Elsevier Ltd. All rights reserved.