The electronic and atomic properties of the interface determine to a large degree the adhesion quality of thin coatings on a material. Since adhesion is promoted by compound formation and atomic intermixing in the interfacial layer, the absence of a stable germanium carbide compound surmises a poor adhesion of a-C:H films on Ge substrates. In contrast, we found a good adhesion of our a-C:H films on Ge which stimulated us to investigate the properties of the interface with photoelectron spectroscopy, high resolution transmission electron microscopy, and ion channeling techniques. We found the formation of a metastable phase and the existence of atomic intermixing in the interfacial layer which clearly demonstrate the reactive nature of the a-C:H/Ge interface. © 1990, The Electrochemical Society, Inc. All rights reserved.