Commercial Nafion (R)-115 (trademark registered to DuPont) membranes were modified by in situ polymerized phenol formaldehyde resin (PFR) to suppress methanol crossover, and SO3- groups were introduced to PFR by post-sulfonatation. A series of membranes with different sulfonated phenol formaldehyde resin (sPFR) loadings have been fabricated and investigated. SEM-EDX characterization shows that the PFR was well dispersed throughout the Nafion (R) membrane. The composite membranes have a similar or slightly lower proton conductivity compared with a native Nafion (R) membrane, but show a significant reduction in methanol crossover (the methanol permeability of sPFR/Nafion (R) composite membrane with 2.3 wt.% sPFR loading was 1.5 x 10(-6) cm(2) s(-1), compared with the 2.5 x 10(-6) cm(2) s(-1) for the native Nafion (R) membrane). In direct methanol fuel cell (DMFC) evaluation, the membrane electrode assembly (MEA) using a composite membrane with a 2.3 wt.% sPFR loading shows a higher performance than that of a native Nafion (R) membrane with 1 M methanol feed, and at higher methanol concentrations (5 M), the composite membrane achieved a 114 mW cm(-2) maximum power density, while the maximum power density of the native Nafion (R) was only 78 mW cm(-2). (c) 2007 Elsevier B.V. All rights reserved.