A new method of water profile determination across the electrolyte of a running H-2/O-2 proton exchange membrane fuel cell (PEMFC) is described. This method, involving in situ small-angle neutron scattering (SANS) technique is based on the modification of the internal structure of the membrane depending on its water content. Prior to the actual experiments, reference spectra have to be recorded in all ranges of water concentration. The profile is then constructed by considering the membrane as a stack of thin polymer layers, each of them representing one single water content. The observed spectrum for the operating PEMFC is, thus considered as a sum of the reference spectra. The calculated coefficient affecting each spectrum is directly correlated to the thickness of the corresponding hydration layer. With this information and the recording of the electrochemical performance of the PEMFC, the temperature and the relative humidity of the gases entering or leaving the cell, it is possible to back calculate the quantity and the origin the water found in the membrane. These SANS experiments showed the inversion of the water concentration profile depending on whether the H-2 sent to the anode was dry or not; it is also showed that even a dried out membrane remains conductive, due to the treatment undergone before being in the PEMFC.