Both pH and pCl sensing plates were fabricated, and the H+ and Cl- distributions in the crevice solution for 18Cr-10Ni-5.5Mn stainless steel were visualized at 0.3 V (vs. Ag/AgCl, 3.33 M KCl) in 0.01 M NaCl (pH 3.0, adjusted with H2SO4) at 298 K. While the pH inside the crevice gradually decreased with time, no accumulation of Cl - ions occurred. The SO42- ions that were added to adjust the pH probably accumulated to maintain electroneutrality. When the pH reached 1.7, a metastable pit was generated inside the crevice, providing a sharp decrease in pH to 0.4 and an increase in Cl- concentration to above 4 M. After that, the corroded area grew steadily with time, and the pH and the Cl- concentration remained constant at pH 0.4 and above 4 M Cl-. Acidification from pH 1.7 to 0.4 was induced by the hydrolysis of the dissolving metal ions from the pit and resulted in the depassivation of the surrounding steel matrix. The metastable pitting also caused the Cl - accumulation inside the crevice by electromigration. The crevice corrosion was found to be initiated when the pH and Cl- concentration inside the crevice reached the conditions for metastable pit formation.