Electrochemical impedance spectroscopy (EIS) is applied as a base diagnostics technique to undertake insitu analysis and categorize irreversible changes in the kinetic and transport properties of a proton exchange membrane (PEM) fuel cell. An extensive study of the performance of an 8 cell-PEMFC with an open air cathode, is carried out under various operating conditions. It is found that the DRT representation complements the equivalent circuit elements approach based on a complex non-linear least square (CNLS) fitting algorithm. It enables a clearer identification of the elementary processes and their dynamic characteristics.