Electrochemical impedance spectroscopy was applied as a suitable technique to characterize the electrolyte/platinum interface of a dye-sensitized solar cell (DSC). The charge-transfer resistance of the electrolyte/platinum interface was examined for different solvents, cations, methods of platinizing the transparent conducting oxide and thicknesses of the platinum layer. One result is that 2 nm of Pt is sufficient for a DSC with acetonitrile as the solvent. Effects of poisoning the catalyst have been discovered. The diffusion constant of IT in the electrolyte was evaluated for different solvents by measuring the diffusion-limited current density. The diffusion overpotential was measured for different bias potentials. At high potentials, a triiodide production in the electrolyte was found. The mechanism of the charge-transfer reaction at the platinum/electrolyte interface was investigated by measuring the dependence of the charge-transfer resistance on the concentrations of iodide and triiodide ions in the electrolyte. The rate-determining step was found to be the electron transfer from I- to the Pt electrode.