When Pt0.25Rh0.75(100) alloy and Pt/Rh(100), Rh/Pt(100) bimetal surfaces were heated in NO or O-2 at T > 400 K, a hybrid surface of Rh-O/Pt-layer/bulk was built up on these surfaces by a chemical reconstruction. The hybrid surface of the Rh-O/Pt-layer gives a characteristic p(3 x 1) LEED pattern. A Pt-deposited Rh(100) surface, Pt/Rh(100), annealed at 1000 K was poorly active for the reaction of NO with H-2, but the reconstructed p(3 x 1)Rh-O/Pt/Rh(100) surface was highly active for this reaction. On the basis of these results, it was concluded that the formation of the Rh-O/Pt hybrid surface during the catalysis is responsible for the prominent activity of the Pt-Rh three-way catalyst.