The system Cu-Au happens to be a classical one that forms continuous solid solution series which, at respective temperatures 425° and 395° C, are dissociated by releasing ordered phases of Cu-Au and Cu₃Au. So far several studies on mutual diffusion were carried out for the systems, which give rise to continuous solid solution series, similarly investigations were made for the systems that form intermetallic compounds (Reaction diffusion). It is established that in the systems belonging to the first type, the diffusion layer is characterised by continuous variation in the composition of diffusion layer. In the second type of systems, layers of intermetallic compounds are formed which are in accordance with the diagram of their state. A conclusion was drawn that the main factor determining the composition and order of layers's growth in diffusion, is the diagram of state for the given system. Difficulty in investigating the diffusion in such systems, lies in this that the oriented phases are formed at temperatures several times lesser than the melting temperature whereas the growth of diffusion layer under these low temperatures proceeds very slowly.