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The diffusion at the Cu/Hf interface has been investigated in situ by the technique of x-ray photoelectron spectroscopy combined with argon ion sputtering. Thick film of hafnium was deposited on a silicon substrate followed by copper. These Cu/Hf samples were annealed at 100, 200, and 300°C for 30 min. The interface width in these samples, as determined from the atomic concentrations, was found to vary linearly with temperature. The interdiffusion coefficients in these samples were determined by the Boltzmann-Matano method. The coefficient is observed to increase with increasing annealing temperature. The Arrhenius plot of the interdiffusion coefficient versus 1/T for 25% atomic concentration of copper was used to determine the activation energy and the pre-exponential factor for this system under the present processing conditions. These values are found to be 0.128 ± 0.003 eV/atom and 3.33×10^-14 ± 1.01×10^-14 cm²/s  respectively.  A comparison with other results indicates that these values depend upon the thickness of the overlayer and the annealing conditions.