: In this study, effect of immersion time on the electrochemical behaviour of AISI 321 stainless steel (AISI 321) in 0.1 M H2S04 solution under open circuit potential (OCP) conditions was evaluated by potentiodynamic polarization, Matt-Schottky analysis and electrochemical impedance spectroscopy (EIS). Matt-Schottky analysis revealed that the passive films behave as n-type and p-type semiconductors at potentials below and above the flat band potential, respectively. Also, Matt-Schottky analysis indicated that the donor and acceptor densities are in the range I 021 cm-3 and increased with the immersion time. E1S results showed that the best equivalent circuit presents two tirn.e constants: The high-medium frequencies time constant can be correlated with the charge transfer process and the low i-equencies time constant has been associated with the redox processes taking place in the sw:face film. According to this equivalent circuit, the polarization resistance (interfacial impedance) initially increases with the immersion time (1 to 12 h), and then ii is observed to decreases. This variation is fidly accordance with potentiodynamic polarization results.