Bimetallic films prepared by consecutive evaporation of Cu and Au onto fused quartz substrates have been annealed for up to 500 h at 160-220°C. The interdiffusion behavior was studied by x rays, using a Seeman-Bohlin diffractometer. The first changes observed were the appearance of Cu 3Au reflections, followed by those from CuAu3. As time progressed, these lines increased in intensity, while those from the parent films decreased without, however, either broadening or changing position. Metallographic observations support the conclusion that interdiffusion produces the layered-structure Cu/Cu3Au/CuAu3/Au with sharp steps in composition between the different phases. Data on relative integrated intensities were used to determine the thicknesses of the new phases as a function of time and temperature. In general, the thickness of CuAu3 was about twice that of Cu3Au, and both phases were found to thicken nearly linearly with time. An activation energy of 1.65 ± 0.17 eV was obtained from the temperature dependence of the thickening rate. The observed kinetics are consistent with a model in which atomic transport through a growing layer occurs rapidly by boundary diffusion, and the rate-limiting step is provided instead by the local atom movements that take place in the crystal lattice behind the advancing interface to secure order and stoichiometry in the new phase.