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Single-phase concentrated solid solution alloys, including high entropy alloys, exhibit remarkable mechanical properties as well as extraordinary corrosion and radiation resistance compared to pure metals and dilute alloys. However, the mechanisms responsible for these properties are unknown in many cases. In this work, we employ ab initio molecular dynamics based on density functional theory to study the diffusion of interstitial atoms in Ni and Ni-based face-centered cubic concentrated alloys including NiFe, NiCo and NiCoCr. We model the defect trajectories over 100 ps and estimate the tracer diffusion coefficients, correlation factors and activation energies. We find that the diffusion mass transport in concentrated alloys is not only slower than that in pure components, i.e. sluggish diffusion, but also chemically non-homogeneous. The results obtained here can be used to understand and predict atomic …