A measurement system based on a symmetrical hydrogen-hydrogen cell was developed and presented. The net water flux through and impedance of Gore (TM) Primea (R) Series 58 MEA were measured under different humidity conditions. An equivalent circuit approach was used to analyze impedance measurement data and the dependences of membrane conductivity and reaction kinetics on the humidity were obtained. Modeling work was found necessary to estimate the diffusion coefficient of water in the membrane, because humidity profile inside the cell was uneven and unknown during the net water flux measurements. Gas permeabilities of different cell components and water uptake of the MEA were measured to get parameter values for the modeling work. The actual modeling work was not conducted in this contribution, but is explained in Part 11 of this paper. (c) 2006 Elsevier Ltd. All rights reserved.