The interdiffusion and intermetallic compound formation of Au/Nb bilayer thin films annealed at 200-450 degrees C have been investigated. The bilayer thin films were prepared by electron beam deposition. The Nb film was 50 nm thick and the Au film was 50-200 nm thick. The interdiffusion of annealed specimens was examined by measuring the electrical resistance and depth-composition profile and by transmission electron microscopy. Interdiffusion between the thin films was detected at temperatures above 325 degrees C in a vacuum of 10(-4) Pa. The intermetallic compound Au2Nb3 and other unknown phases form during annealing at over 400 degrees C. The apparent diffusion constants. determined from the penetration depth for annealing at 350 degrees C, are 3.5 x 10(-15) m(2) s(-1) for Nb in Au and 8.6 x 110(-15) m(2) s(-1) for Au in Nb. The Au surface of the bilayer films becomes uneven after annealing at over 400 degrees C due to the reaction.