The structure of hydroxylated oxide films (passive films) formed on Cr(110) in 0.5 M H2SO4 at +0.35, +0.55, and +0.75 V/SHE has been investigated by in situ scanning tunneling microscopy (STILI). Cathodic reduction pretreaments at -0.54, -0.64, and -0.74 V/SHE destroy the well-defined topography of the single crystal electrode and they have been excluded from the passivation procedure. Two different passive film structures have been observed, depending on the potential and time of passivation. At low potential (+0.35 V/SHE), the passive film, consisting mostly of chromium hydroxide, has a noncrystalline and granular structure whose roughness suggests local variations of thickness of ca. rt 0.5 nm. A similar structure is observed at higher potential (+0.55 V/SHE), but only for a short polarization time. For longer polarization at 0.55 V/SHE, and at higher potentials (+0.75 V/SHE), a crystalline structure is formed; the higher the potential, the faster the crystallization. It corresponds to the growth of a chromium oxide layer in the passive film. This chromium oxide laver is (0001) oriented. A structural model of the passive film is proposed, with termination of this oxide layer by a monolayer of hydroxyl groups or of chromium hydroxide in (1 x 1) epitaxy with the underlying oxide, and with surface steps resulting from the emergence of stacking faults of the Cr3+ planes in the oxide layer. Energy band models of the electronic structure of the semiconductive passive films show that the tunneling mechanism of the STM imaging involves empty electronic states located in the band gap of the passive film. The growth of the oxide layer in the passive film is governed by a combined reaction of dehydration of chromium hydroxide and oxidation of chromium: Cr(OH)(3) (film) + Cr (metal) --> Cr2O3 (film) + 3 H+ + 3 e(-).