A detailed picture of the thermally activated processes occurring at the Co/ZnO interface is obtained by a combination of high energy X-ray based techniques: X-ray photoelectron and absorption spectroscopies and the kinematical X-ray standing wave method. At room temperature, the growth of a few monolayers of cobalt proceeds by the nucleation of nanometer-sized clusters on the polar oxygen-terminated (0001̄) surface of a ZnO single crystal. Progressive annealing from 600 to 970 K allows separating the various interfacial reactions. At the lowest annealing temperature, Co clusters coalesce while keeping their metallic character. Above 700 K Co is gradually oxidized to Co and a thin Co rich (Zn,Co)O layer is formed. It is observed that rock salt CoO phases may form at the surface when the initial Co thickness exceeds 1 nm. At the highest annealing temperature (970 K), Co diffuses deeper into ZnO and Zn vacancies are created at subsurface sites that were previously occupied by Co.