Passive, stand-alone, direct methanol fuel cells require a pressure management system that releases CO2 produced in the anode chamber. However, this must be done without allowing the methanol fuel to escape. In this paper, two siloxane membranes are investigated and shown to selectively vent CO2 from the anode chamber. The addition of hydrophobic additives, 1,6-divinylperfluorohexane and 1,9-decadiene, improved the selectivity of the siloxane membranes. The best performing CO2 vent was obtained with 50:50 wt% poly(1-trimethyl silyl propyne) and 1,6-divinylperfluorohexane. (C) 2008 Elsevier B.V. All rights reserved.