The impedance of each cell of a 10 W PEMFC stack under conditions of natural running is determined independently of the others. No electronic load is used during the experiments, even if the current flowing through the stack is greater than the current range of our electrochemical apparatus. Only a power resistor is needed, which is simpler and cheaper to use. It is shown that cell by cell, a gas starvation occurs in the stack when H-2 and O-2 circulate in cocurrent, the impedance increasing from the cell which is on the gas inlet-side to the cell which is on the gas outlet-side. It is also shown that the part of the internal resistance of a cell which is due to the polymer membrane is similar to40%. The method proposed in this paper can be applied whatever the number of cells in the stack, and could be useful in order to detect a failure due to only one cell, whereas the others run well. (C) 2004 The Electrochemical Society.