Nanoparticles of rhodium oxides were prepared in the MCM-41 mesoporous molecular sieve and were characterized by XRD, TEM. H2-TPR and EXAFS. It was found that the particle size, location and structure of the rhodium oxide particles were greatly influenced by the hydrothermal synthesis conditions. The Rh K-edge EXAFS study revealed that the nanoparticles of RhOx, with the size <3 nm has a hexagonal structure with a Rh-Rh bond distance of 0.273 nm, while the nanoparticles of RhOx with 6-8 nm size possessed a distorted orthorhombic structure without any long range order. The RhOx nanoparticles were employed as catalysts forNO-CO reactions. The rhodium oxides with <3 nm particles located in the mesopore channels of MCM-41 exhibited a promoting effect of excess O-2 on the conversion of NO to N-2 and N2O in the catalytic NO-CO reaction. The rhodium oxides of 6-8 nm size located in the bulk of MCM-41 and a supported RhOx/MCM-41 catalyst showed poisoning effects of excess O-2 on the NO and CO conversions. The promotional effects of O-2 observed for the catalyst with RhOx nanoparticles in the mesopores were attributed not to the small size of RhOx, but to the location of RhOx in the MCM-41 channel.