When Pt-Rh alloy or Pt/Rh bimetallic surfaces were exposed to O-2 or NO, Rh atoms underwent segregation at temperatures higher than ca. 400 K, and was followed by the characteristic restructuring. That is, Pt-Rh(100) alloy, Rh/Pt(100) and Pt/Rh(100) bimetallic surfaces commonly form a p(3 x 1) structure by heating in O-2, and Pt/Rh(110) and Rh/Pt(110) surfaces heated in O-2 give c(2 x 4) and c(2 x 2) LEED pattern, respectively. interestingly, the catalytic activity of the bimetallic surfaces for NO + H-2 reaction was markedly enhanced by the reconstruction and became structure insensitive. An STM image in atomic resolution revealed an ordered Ph and Pt composite structure for the p(3 X 1) Pt-Rh(100)/O alloy surface. These results suggest that the reconstructed bimetallic surfaces may be composed of the active sites, having a common local structure.