Dependences of the parameters of interdiffusion (effective coefficients, diffusion paths, Kirkendall shift) on the variation of the initial compositions of a diffusion couple have been studied on the concrete example of the Co–Fe–Ni system. The experimental results obtained show that the diffusion paths in the three_component system depend substantially on the selection of initial conditions, even when the initial alloys of the "narrower" diffusion couple lie on the diffusion path of the "wider" pair. At the same time, when represented in reduced concentrations, the diffusion paths of all these diffusion couples practically coincide. The latter makes it possible to predict the type of diffusion paths for any diffusion couple whose initial composition lies in the same concentration region. In one of the diffusion couples studied, the instability of the Kirkendall plane was revealed, which thus far was observed only in binary systems.