This work describes the effect of polyaniline intercalations, produced by polymerization in situ of aniline in perfluorinated sulfocationic polymer templates, on the electrochemical properties of the resulting composite membrane. During the polymerization in situ process, the color of the template evolves rapidly from white to blue and then to emerald green. After 5 h of aniline polymerization, both pristine and composite membranes exhibit higher protonic conductivity than aqueous sulfuric acid solutions with concentrations lower than 0.1 M. After a polymerization time of 30 days, intercalations made up of a mixture of emeraldine and pernigraniline are formed that reduce the conductivity to one-third of that corresponding to the pristine acid membranes. The electroosmotic water transport number across both the pristine and composite acid membranes is about 3 mol/Faraday, a rather low value explained by the high protonic conductivity of the membranes. Apparent cation transport numbers determined from electromotive forces of concentration cells are little affected by the polyaniline intercalations and their values are rather close to those calculated taking the membranes as reference frame. (c) 2008 Elsevier B.V. All rights reserved.