The thermal properties of phosphoric acid-doped poly[2-2'-(m-phenylene)-5-5' bi-benzimidazole] (PBI) and poly[2,5-benzimidazole] (ABPBI) membranes, ionomeric materials with promising properties to be used as electrolytes in direct methanol and in high temperature polymer electrolyte membrane (PEM) fuel cells, were studied by means of differential scanning calorimetry (DSC) technique in the temperature range from -145 degrees C to 200 degrees C. The DSC scans of samples equilibrated in water at different relative humidities (RH) and in liquid water methanol mixtures were analyzed in relation to glass transition, water crystallization/melting and solvent desorption in different temperature regions. The thermal relaxation observed in the very low temperature region could be ascribed to the glass transition of the H3PO4-H2O mixture confined in the polymeric matrix. After cooling the samples up to 145 degrees C, frozen water was detected in PRI and ABPBI at different RH, although at 100% RH less amount of water had crystallized than that observed in Nation membranes under the same conditions. Even more important is the fact that the freezing degree of water is much lower in ABPBI membranes equilibrated in liquid water methanol mixtures than that observed for PRI and, in a previous study, for Nafion. Thus, apart from other well known properties, acid-doped ABPBI emerges as an excellent ionomer for applications in direct methanol fuel cells working in cold environments. (C) 2010 Elsevier B.V. All rights reserved.