High kinetic-energy photoelectron spectroscopy (HIKE) or hard x-ray photoelectron spectroscopy has been used to investigate the alloying of Ni/Cu (100) multilayers. Relative intensities of the corelevels and their chemical shifts derived from binding energy changes are shown to give precise information on physicochemical properties and quality of the buried layers. Interface roughening, including kinetic properties such as the rate of alloying, and temperature effects on the processes can be analyzed quantitatively. Using HIKE, we have been able to precisely follow the deterioration of the multilayer structure at the atomic scale and observe the diffusion of the capping layer into the multilayer structure which in turn is found to lead to a segregation in the ternary system. This is of great importance for future research on multilayered systems of this kind. Our experimental data are supplemented by first-principles theoretical calculations of the core-level shifts for a ternary alloy to allow for modeling of the influence of capping materials on the chemical shifts.