A detailed picture of the thermally activated processes occurring at the Co/ZnO or Mn/ZnO interfaces is obtained by a combination of a wide range of techniques. The low energy electron diffraction, the scanning tunnelling microscopy and spectroscopic techniques based on Auger electron, on high energy X-ray: X-ray photoelectron and absorption spectroscopies and on the kinematical X-ray standing wave method allow not only to investigate the interaction at the heterojunction but also to describe in detail the thermal structure transformations and diffusion process. At room temperature, the growth of a few monolayers of the transition metal M (M = Co or Mn) proceeds by the nucleation of 3D nanometer-sized clusters on the polar flat surface of a ZnO single crystal. Progressive annealing up to ~1000 K allows separating the various interfacial reactions. At the lowest annealing temperature, M clusters coalesce while keeping their metallic character. The thin film is gradually oxidized to M2+ and a thin M rich ZnxM1 xO layer is formed. For M = Co, it is observed that rocksalt CoO phases may form at the surface when the initial Co thickness exceeds 1 nm. For M = Mn, upon annealing to progressively higher temperature ranging from 575 K to 800 K, various Mn oxides phases form successively. At first, the ZnxMn1 xO layer forms on top of the surface. Then, while Mn atoms diffuse deeper into the bulk of ZnO, ZnyMn3 yO4 starts to form at the surface. Finally, at the highest annealing temperature, Mn and Co appears to be substitutionally diluted at Zn sites within the ZnO lattice.