Results of an in situ X-ray scattering study of the passive film formed on Ni(111) electrodes by passivation in 0.05 M H2SO4 (pH 1.0) at 0.50 V-Ag/AgCl are reported and compared with results on the film formed by oxidation in air at room temperature. Tn both cases, ultrathin, (111)-oriented NiO films are observed, which are aligned with the Ni substrate lattice and slightly expanded along the surface normal with respect to bulk NiO. However, two major structural differences are found: (i) while on the air-formed oxide parallel (NiO[1 (1) over bar 0] \\ Ni[1 (1) over bar 0]) and antiparallel (NiO[1 (1) over bar 0] \\ Ni[(1) over bar 10]) oriented domains coexist; the passive film exhibits. a well-defined antiparallel orientation and (ii) the lattice of the passive film is, in contrast to that of the air-fomed oxide, tilted relative to the substrate with a broad angular dispersion of the tilt angle centered at about 3.3 degrees.