The composition, stability, and thermal evolution of nitric oxide on Rh(100) have been characterized by means of synchrotron radiation fast x-ray photoelectron spectroscopy and low-energy-electron diffraction. At 140 K, there is a single NO chemisorbed species (alpha(2)-NO) at saturation, forming a disordered phase which develops in an ordered p(4root2xroot2)R45degrees structure after heating the layer to 373 K. A further annealing induces desorption and a progressive dissociation of the NO molecules, which is completed above 425 K, where a c(2x2) phase consisting of coadsorbed N and O atoms is observed. At low NO coverage, a different NO molecular species (alpha(1)-NO), characterized by approximate to1.7 eV lower N1s binding energy, is detected on Rh(100). alpha(1)-NO is observed to undergo dissociation under the photon beam. The N1s line shape modifications due to irradiation by 500 eV photons were monitored in real time by using fast photoemission. The different stability of the alpha(1)-NO and alpha(2)-NO species is discussed. (C) 2003 American Institute of Physics.