The strain profile of annealed Cu-Ni multilayers was analysed using an X-ray diffraction (XRD) theory. The annealing times of the multilayers ranged from 0 to 20 h. The strain in each layer was found by fitting the theoretical peak intensities with the experimental ones by iteration and using a kinematical/dynamical theory of XRD. It was found that for increasing annealing times, there was a decrease in the strain profile due to increased interdiffusion between the Cu and Ni layers. The increase in diffusion changed the composition modulation of the multilayers progressively from a trapezoidal wave for the 0 h annealed sample to a sinusoidal wave for the 20 h annealed sample.