Alkaline doped PBI membranes are investigated for their physicochemical properties when employed in alkaline direct alcohol fuel cells. Alkali uptake, water uptake, alcohol permeability, ion conductivity, mechanical strength and evolution of the morphology for the alkaline doped PBI membranes are measured. It is demonstrated that the alkaline doping process has a profound influence on the physicochemical properties of PBI membranes. With a high alkaline doping level, the alkali uptake of alkaline doped PBI membranes reaches 14%. The high alkaline uptake results in a separation of the polymer backbones and partial cleavage of hydrogen bonds, which leads to a reduction in the mechanical strength and an increase the alcohol permeability of alkaline doped PBI membranes. However, the high alkaline doping level is beneficial to the ionic conductivity. The conductivity of alkaline doped PBI membranes increases to 96.1 mS cm(-1) at 363.15 K after PBI membranes are doped with 6 M KOH solution. Based on these investigations, a mechanism is proposed to interpret the chemical interaction during the alkaline doping process and its influence on physicochemical properties. The existence of a neutralization reaction and the re-establishment of hydrogen bonding networks should be attributed to the property transformation during the alkaline doping process. (C) 2015 Elsevier B.V. All rights reserved.