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A phenomenological model was utilized to describe diffusivities in the c (fcc) ϒ (ffc)/ ϒ’ (L1₂) and A2/B2 phases of the Ni–Al system. An effective strategy, which takes the homogeneity range and defect concentration into account, was developed in the present work to optimize the atomic mobilities of ϒ’ phase. Such a strategy results in a dramatic decrease in the number of atomic mobility parameters to be evaluated for the L1₂ phase. The measured composition- and temperature-dependent diffusivities in the Ni–Al system have been well replicated by the present mobility descriptions. For the L1₂ phase, comprehensive comparisons show that with fewer model parameters the presently obtained mobilities yield a better fit to experimental diffusivities, compared with previous assessments. The mobility descriptions are further validated by comparing calculated and measured concentration profiles for various diffusion couples. The time-dependent Al composition profile for the annealed vapor Al/ϒ couple is accurately described for the first time.