Data corresponding to the isotherms for the equilibrium water content of a PolyBenzImidazole (PBI)-H3PO4 High Temperature Membrane Electrode Assembly (HT-MEA) are presented over a range of temperatures from 30 degrees C to 180 degrees C. The data show values for the ratio of water to acid which range from 4.0 at 70 degrees C to 0.2 at 160 degrees C at 30 kPa of water vapor pressure. In addition to the equilibrium data, rates of adsorption are shown to have time constants on the order of 500 s and 2500 s at 70 degrees C and 50 degrees C, respectively. At temperatures above 100 degrees C the time constants for adsorption are on the order of 100 s. These data allow for the calculation of "acid dilution" in this HT-MEA during low-temperature excursions and start up/shut down. (C) 2010 Elsevier B.V. All rights reserved.