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This work presents experimental and computational research on the influence of hot isostatic pressing (HIP) of CMSX-4, a second-generation single-crystal nickel-based superalloy, on interdiffusion behavior and the formation of Kirkendall voids in subsequently assembled diffusion couples under isothermal annealing. Two sets of Ni/CMSX-4 diffusion couples were prepared: one using CMSX-4 that had undergone HIP treatment, and the other using CMSX-4 in the as-cast condition. The diffusion couples were annealed at 1250 °C for 144 h. Experimentally measured interdiffusion profiles of all alloying elements, together with synchrotron X-ray 3D μ-tomography of Kirkendall voids, were compared with calculated diffusion profiles and phase-field simulations of void evolution. A direct comparison between the simulation and experimental results enables an explanation of the HIP effect on the density, distribution, and size of the voids, as well as on the observed shifts in the interdiffusion profiles.