In this work, the influence of several parameters that seem to affect the polycondensation synthesis of PBI has been studied. A significant effect of the dosage of polyphosphoric acid on the resulting molecular weight of PBI was observed. Conversely, the addition of drying agent (P2O5) or catalyst (triphenyl phosphate) appeared not to be beneficial. Likewise, temperature increased strongly the molecular weight of PBI and reaction times around 9 h seemed to be long enough to produce a high quality PBI. Membranes cast from high molecular weight PBIs showed enhanced mechanical resistance (higher stress at break), and chemical resistance (lower degradation rate in the Fenton test) and a high thermal stability even after the hydrogen peroxide treatment. Films mechanical properties seriously deteriorated as doping level went up. However, high molecular weight PBI membranes still maintained acceptable mechanical properties even when impregnated with massive amounts of acid (doping level of 14.5). Those high doping levels implied a significant increase in the conductivity and a much enhanced cell performance, which did not rely on the polymer molecular weight. © 2007 Elsevier B.V. All rights reserved.