Pitting corrosion investigations of 304L stainless steel in 0.5 M NaCl have been performed by means of electrochemical impedance spectroscopy (EIS) under linear potential change conditions. The general electric equivalent circuit has been determined in a wide range of potentials, covering the passive state, the metastable development of pits and spontaneous pit formation. It has been shown that impedance measurements in potentiodynamic conditions allow simultaneous investigation of changes in the passive layer structure, ion transport processes through the passive layer and electrode process kinetics inside the pits. It has been stated that pitting corrosion in the metastable range is activation controlled. In the potential range of the spontaneous pit development electrode processes inside the pits are also activation controlled. (C) 2004 Elsevier Ltd. All rights reserved.