Electrochemical impedance spectroscopy (EIS) was used to understand the electrochemical mechanisms which appear in dye-sensitized solar cells (DSSCs). This qualitative and quantitative technique permits identification of the phenomena proceeding within the different elements composing the cell and at their interfaces. In this study, the classical conducting glass substrate was replaced by a protected stainless steel (304 type) substrate as the counter-electrode (cathode) in dye-sensitized solar cells. Platinum was deposited at the substrate surface to optimize the charge transfer resistance of the electrode. After a few days of immersion in the electrolytic solution, stainless steel substrates coated with low thickness of Pt show pitting corrosion due to iodine. Defects in the Pt layer such as discontinuity of the film and micro-cracks may explain the corrosion of the stainless steel substrate. However the Pt layer degradation is retarded for thicker films. On the other hand, polished substrates show a better behaviour probably due to the elimination of the defects on the stainless steel surface. Electrolytic solution was optimized. For this, components such as 1-butyl-3-methylimidazolium iodide (BMII), guanidine thiocyanate (CT) and 4-tert-butylpyridine (TBP) were added. No corrosion phenomena on stainless steel 304 appeared within 3 days when TBP was added. This means that TBP acts as a corrosion inhibitor. A schematic equivalent circuit is also proposed. (C) 2011 Elsevier Ltd. All rights reserved.