A detailed experimental and theoretical investigation of the structure of nitric oxide adsorption layers formed at 140 K on Rh(100) has been carried out by means of x-ray photoelectron spectroscopy, x-ray photoelectron diffraction, near-edge x-ray absorption fine structure, and ab initio calculations. At saturation, a single NO species is present. Both theory and experimental results indicate a bridge adsorption site for NO in this phase, with the molecules standing upright on the surface. At low NO coverage, the presence of a different molecular species is experimentally revealed and it is fully characterized by ab initio calculations. This species has been identified with a NO molecule with the molecular axis almost parallel to the surface, lying above a Rh(100) hollow site with the two atoms in asymmetric bridge sites. (C) 2003 American Institute of Physics.