A new method for measuring separately anode and cathode impedances (Z(A) and Z(C)) of an electrochemical generator running on constant load is proposed. In these conditions, the measurement of Z(A) and Z(C) is not as simple as under galvanostatic conditions. Measurements are carried out first on a dummy cell and next on a commercial direct methanol fuel cell (DMFC) under galvanostatic conditions and using the new proposed method. The comparison of the results validate our method, showing that it is a powerful method for fuel cell electrochemical impedance spectroscopy (EIS) characterization under natural running conditions.