This research focuses on the surface modification of a perfluorosulfonic acid (PFSA) membrane using plasma treatment. The plasma gas was a mixture of perfluoroheptane (C7F16) and argon (Ar). The results indicated that the plasma mixture led to the deposition of a thin fluorocarbon polymerized top layer onto the PFSA membrane. The effects of plasma operating conditions (plasma power, reactor pressure, and treatment time) on the product properties were analyzed using a factorial design. The radio-frequency (RF) power was the most influencing factor. The film deposition rate, surface roughness, and cross-linking of the top fluorocarbon film increased with RF power, associating with a higher contact angle and lower methanol permeability in the resulting membrane. Higher RF power also resulted in a decreased ion exchange capacity and water uptake in the membranes, mainly due to the lack of sulfonate functional groups and the cross-linked fluorocarbon top layer. However, the number of water molecules associated with each sulfonate group did not deviate from that of the pristine membrane, indicating that the water cluster size was not significantly altered by the plasma modification process. The effective electrical conductivity contribution from the deposited top layer was at least five orders of magnitudes higher than other fluorocarbon film. Meanwhile, the methanol permeability of the treated membrane rendered a two-order magnitude decrease, which was ascribed to the combined effect of the more hydrophobic surface and the higher level of cross-linking in the deposited fluorocarbon top layer. The suppressed methanol crossover in the modified membranes offers advantages for direct methanol fuel cell operations at high methanol concentrations. (c) 2007 Elsevier B.V. All rights reserved.