The adsorption states and thermal reactions of NO on the clean and nitrogen modified Rh(111) surfaces were investigated between 20 and 150 K using infrared reflection adsorption spectroscopy (IRAS) and temperature programmed desorption. On the clean surface, singleton species at atop and hollow sites were observed at 1816 and 1479 cm(-1), respectively. Using time-resolved IRAS, the activation energy and pre-exponential factor of the site change from atop to hollow sites on Rh(111) were estimated to be 117 meV and 1.7x10(10) s(-1), respectively. On the saturated monolayer, physisorbed NO dimers were formed. In the second layer, they were adsorbed with the N-N bond nearly parallel to the surface. In the multilayer formed at 20 K, the NO dimers were randomly oriented. On the nitrogen modified Rh(111) surface, a new adsorption state of chemisorbed monomer was observed as well as atop and hollow species. Physisorbed NO dimers were a precursor to N(2)O formation on the nitrogen modified Rh(111) surface. In the N(2)O formation reaction, three kinds of N(2)O species were identified. The first species desorbed from the surface immediately after the formation reaction, which is a reaction-limited process. The second species was physisorbed on the surface and desorbed at 86 K, which is a desorption-limited process. The third species was chemisorbed on the surface and decomposed above 100 K.