The impedance measurements in a highly resistive electrolyte medium are difficult. The use of two identical electrode cells having a large surface area may overcome this difficulty, but measurements have to be restricted to the corrosion potential. The advantages of each type of cell are discussed. The electrochemical system tested in this work was the corrosion of mild steel in an automotive fuel ethanol with or without ethanolamines as corrosion inhibitors. It was found that in a three electrode cell, the use of a special electronic circuit increasing the input impedance of reference electrode was necessary to perform correct measurements. No particular care was needed in the impedance measurements in a two electrode cell. If the two electrode plates were set too close, the measured impedance was significantly great, thus leading to an erroneous prediction of the corrosion behaviour. A tentative explanation to this phenomenon is given.