We study the interdiffusion and formation of Al3Ti, in bimetallic thin films of Al/Ti and (Al0.25 at. % Cu)/Ti, both with and without W diffusion markers, in the temperature range 350500°C. The growth kinetics of Al3Ti and marker displacement were measured by Rutherford backscattering spectroscopy. Complementary structural and compositional information were obtained by glancing-incidence x-ray diffraction, transmission electron microscopy, Auger-electron spectroscopy, and secondary-ion-mass spectroscopy. The effect of Cu, revealed by marker analysis of the intrinsic diffusivities of Al and Ti in Al3Ti, is to increase the activation energy of Ti diffusion from 1.68 eV to 2.17 eV while the diffusion of Al is much less affected. By examining the crystal structure of Al3Ti, a vacancy mechanism with Cu occupying Al sites is proposed to explain the solute effect.