Polybenzimidazole (PBI) film, a candidate polymer electrolyte membrane (PEM) for high-temperature (120-200degreesC) fuel cells, was cast from PBI/trifluoacetyl/H3PO4 solution with constant molecular weight PBI powder and various acid doping levels. Conductivity measurements on these membranes were performed using an ac method under controlled temperature and relative humidity (RH). A complete set of conductivity data for H3PO4 acid-doped PBI is presented as a function of temperature (60-200degreesC), RH (5-30%), and acid doping level (300-600 mol %). A mechanism of conductivity is proposed for the proton migration in this PBI/acid system based on this and previous work. Proton transfer in this system appears to occur along different paths for different doping levels, RHs, and temperatures. Hydrogen bonds immobilize the anions and form a network for proton transfer by a Grotthuss mechanism. The rate of proton transfer involving H2O is faster, leading to higher conductivity at higher RH. The order of the rate of proton transfer between various species is H3PO4(H2PO4-)...H-O-H>H3PO4...H2PO4-. N-H+...H2PO4-, N-H+...H-O-H>N-H+...N-H. The upper limit of proton conductivity is given by the conductivity of the liquid state H3PO4. (C) 2003 The Electrochemical Society.