Interdiffusion in Cu–Al thin film bilayers at temperatures between 160 and 300 °C has been studied by a combination of glancing‐incidence x‐ray diffraction, Rutherford backscattering spectroscopy, and transmission electron diffraction and microscopy. A sequential intermetallic compound formation was observed in samples with an excess amount of Cu with &thgr;‐CuAl2forming first, followed by &eegr;2‐CuAl, and &ggr;2‐Cu9Al4. In samples with excess Al, the &thgr;‐CuAl2is the first and the last phase formed. The thickening of these compounds was found to obey a parabolic relationship with time, and especially the thickening of &thgr;‐CuAl2can be described by a prefactor of 7.4 cm2/s and an activation energy of 1.31 eV.