In situ surface enhanced Raman spectroscopy was employed to study the effect of dissolved oxygen (DO) an the composition of the corrosion film formed on a low carbon steel surface in 0.01 M NaCl solution. Raman spectra were taken during cyclic voltammetric and potential step experiments. The spectra taken during cyclic voltammetry were similar to those previously obtained for passive iron. It showed a peak for a trivalent species at 670 cm(-1) in the passive potential range, which was usually assigned to FeOOH rather than to gamma -Fe2O3. However, in the spectra taken during potential step experiments, it was apparent that the main trivalent species in the corrosion film was gamma -Fe2O3 at 640, 670, 715 cm(-1), and DO behaved as an oxidizer to convert iron from the divalent stair in Fe3O4 to the trivalent state (gamma -Fe2O3). The presence of gamma -Fe2O3 in the corrosion film on iron was detected for the first time bq taking SER spectra during potential step experiments. Though this film showed a weak protective property, and had corrosion products due to pitting induced by chloride ions, the detection of gamma -Fe2O3 supports the previous ex situ and in situ findings that the trivalent oxide in iron passivity is gamma -Fe2O3.