This paper describes a novel diagnostic test method that provides insight into the distribution of liquid water in a fuel cell under operation. The amount and distribution of liquid water in a fuel cell is measured indirectly through the measurement of certain physico-chemical properties which are functions of the liquid water content. The MEA resistance and electrode diffusivity (MRED) method allows the measurement of (i) the pure ohmic cell resistance, (ii) the effective diffusivity of the electrodes and (iii) the free gas volume of the fuel cell. The pure ohmic resistance enables the determination of the membrane water content whereas the free gas volume measurement allows the amount of liquid water in both the anode and cathode compartments to be deduced. The MRED method was used to study the pure ohmic resistance distribution during a drying/re-hydration cycle and the effective oxygen diffusion coefficient of the cathode electrode. Furthermore, the accuracy of the liquid water volume determination was investigated. (c) 2004 Elsevier B.V. All rights reserved.