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Interstitial alloying is nowadays an important direction for further developments of High Entropy Alloys. In this work, the impact of interstitial carbon on the thermodynamics–kinetics coupling in CoCrFeMnNi-based HEAs is studied through CALPHAD and continuum approaches. First, purely thermodynamic characteristics (phase diagrams and chemical potentials) are calculated. Then purely kinetics properties, i.e., carbon-content dependent mobilities of substitutional elements are modeled, based on experimental data. Consequently, a continuum model is applied to combine these properties in order to simulate interdiffusion in carbon-free/carbon-bearing HEA couples, underlining mutual influence of carbon and matrix elements diffusion on each other. The results are benchmarked against the experimental composition profiles and a very good agreement is observed, specially when carbon effect for all elements is explicitly included. A non-monotonous dependence of the chromium up-hill diffusion on the carbon content is explained by a strong thermodynamic interaction between carbon and chromium elements.