Quantitative structural analysis by conventional X-ray diffraction and anomalous X-ray scattering coupled with the reverse Monte-Carlo simulation technique has been carried out, in order to characterize the atomicscale structure of rust formed on the surface of Fe, Fe-2 %Cr, Fe-3 %Ni and Fe-1.6 %Cu alloys and a weathering steel. Rust samples were prepared by filtering corrosion products formed on the surface of these alloys in salt water. X-ray diffraction patterns showed that main components of the rust consist of ferric oxyhydroxides such as γ-FeOOH. Radial distribution functions obtained from both conventional X-ray diffraction and anomalous X-ray scattering clearly indicated that the fundamental local unit structure in the rust is the octahedral FeO₆, although it is likely to be distorted. Realistic atomic arrangements in the rust were estimated by fitting of the ordinary and environmental interference functions calculated using the reverse Monte-Carlo simulation technique. These results provided that the network structure consists of the FeO₆ octahedral units and its distortion depend upon the composition of samples. These facts suggest that alloying elements in the rust, which themselves are corroded, play an important role in forming the atomic-scale structure of the rust.