The acid migration in phosphoric acid-doped polybenzimidazole (PBI) membrane high-temperature proton exchange membrane fuel cells (HT-PEMFC) during operation is experimentally evaluated to clarify the influence of the acid balance between the membrane and electrodes on cell performance. A method for controlling the amount of phosphoric acid doped in PBI membranes is investigated, and PBI membranes with various amounts of phosphoric acid are prepared. Cell operation tests and AC impedance spectroscopy of cells fabricated with these membranes are conducted. It was found that the amount of phosphoric acid doped in the membranes can be controlled by changing the solution temperature and the immersion time in phosphoric acid solution. It was also found that the HT-PEMFC performance can be improved by optimizing the amount of phosphoric acid doped in the membrane and by diffusion of phosphoric acid into the catalyst layer during the initial stage of cell operation. (C) 2008 Elsevier B.V. All rights reserved.