The modification of Nafion membranes with polyaniline (PAni) has been studied as an alternative for reducing methanol crossover in direct methanol fuel cells (DMFC). The modification has been performed by directly polymerizing the PAni following two different routes: immersion (Naf-S-Y, where S mean surface and Y the number of hours exposition) and crossover (Naf-C-Y, where C means crossover). The former consist of exposing the membranes to a reactive solution containing the aniline, oxidant and catalyst; while in the latter the aniline and a solution with the oxidant and the catalyst are in different chambers separated by the membrane, thus forcing them to react inside it. The effect of the modification mechanism and the reaction times has been studied. The resulting membranes were extensively characterized by means of Fourier Transform Infrared (FTIR), ionic exchange capacity (IEC), water uptake (WU), methanol permeability and single direct methanol fuel cell performance. Chemical characterization revealed that the oxidation state of the polyaniline was in all cases emeraldine and the amount of PAni for an equivalent exposure time was bigger for the crossover route. The crossover route has proven to be more effective in decreasing the apparent methanol permeability of Nafion modified membranes up to 48% for the crossover sample with higher modification time when the polymerization is due inside the membrane such is the case of the composite Naf-C-Y membranes. The Direct Methanol Fuel Cells performances of membrane-electrode assemblies prepared with pristine Nafion and Nafion-PAni membranes were tested at 40, 60 and 80 degrees C under 2 M methanol concentration. The results are compared with those found for Nafion pristine membranes which power densities were 90, 65, 60 and 50 mW/cm(2) at 80 degrees C for Nafion, Naf-S-1, Naf-S-5 and Naf-C-2, membranes respectively. (C) 2015 The Electrochemical Society. All rights reserved.