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A detailed study has been made of the X-ray characterization of Cu/Ni multilayers. Different mathematical models have been used to calculate the X-ray diffraction intensity and theoretical and experimental profiles were compared. An extended step model, which incorporates repeat-period fluctuations, and a model for interdiffusion were used. By varying the repeat-period fluctuation, the theoretical X-ray profile obtained from the extended step model was compared with the experimental curve. A best fit was obtained for a 1.6% fluctuation in the repeat period. For the second model, Cu/Ni samples annealed for different times (i.e. 2, 6, 14 and 20 h) were considered. The experimental values of the amplitudes of the first and second harmonics of the interplanar spacing were compared with the theoretical values. A coherent interface is predicted for the un-annealed sample. It is observed that the longer the annealing time, the higher is the interdiffusion. The interplanar spacing of the multilayers approaches the average interplanar spacing of the homogeneous Cu/Ni mixture with the longer annealing time (i.e. 20 h).