Abstract: The pitting mechanism of passive 304 stainless steel monocrystal in pH1.0, 0.25mol/l Na₂SO₄ solutions containing chlorion has been investigated using a. c. impedance technique. The impedance characteristics in pit initiation and propagation stages have been analysed. It is proposed that an intermediate complex (MOHCI)comc will form in both the two stages. A pitting mechanism model and an equivalent circuit model in pit propagation stage have been propesen, it has no intrinsic difference with that for 304 ss polycrystal.