This work summarizes results on the modification of perfluorinated sulfocationic membranes MF-4SC by in situ chemical polymerization of aniline. The investigation of transport properties of polyaniline/MF-4SC composite membranes after bulk modification -conductivity, diffusion and electroosmotic permeability, proton permselectivity - as well as porosimetry and polarization behavior is carried out as functions of aniline polymerization parameters and acid concentration. The fibrous-cluster model of a composite membrane is proposed for the estimation of transport and structural parameters, taking into account different mechanism of charge transfer in structural fragments of the composite. The atomic force microscopy images and curves of water distribution on the effective pore radii in the composite membranes testify to a morphological transition from the nano- to the microsize of polyaniline inclusions with increasing the aniline polymerization time. This effect is confirmed by the analysis of two-phase model transport and structural parameters. High values of the "true" proton transport numbers of composites are obtained and discussed. The dynamic hydration numbers of protons and chloride co-ions are estimated using the "true" transport numbers of protons and the electrocismotic coefficients of composites. The current-voltage curves of composite membranes in the "free standing" state after bulk and surface modification by polyaniline are investigated. The effect of stabilization of limiting current density is observed for MF-4SC membrane after bulk modification. The effect of current-voltage curves asymmetry is observed for different orientation of the polyaniline layer towards the current direction for an anisotropic composite membrane after surface modification. (C) 2008 Elsevier Ltd. All rights reserved.