Phosphoric acid-doped membranes based in poly[2,5-benzimidazole] (ABPBI) were obtained by a new low temperature casting procedure and by the classical high temperature casting from methanesulfonic acid. These membranes, which can be suitable for application in direct methanol proton exchange membrane (PEM) fuel cells, were studied in relation with their phosphoric acid doping level by measuring the free and bonded acid. The water isotherms were also determined for the low and high temperature casted ABPBI membranes. Both, acid and water sorption properties, were compared with those determined in poly [2-2'-(m-fenylene)-5-5' bibenzimidazole] (PBI) membranes, The water sorption of the ABPBI membranes over the range of all water activity is described by the modified BET equation. commonly known as Guggenheim-Anderson-de Boer (GAB) and a two-parameters empirical isotherm. The acid uptake behaviour of the membranes prepared by low and high temperature casting are related with differences in their supramolecular structure. (C) 2008 Elsevier B.V. All rights reserved.