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The phase equilibria, diffusion growth and diffusivities in the Ni-Al-Pt system at 1150, 1 200 and 1 250 °C were studied using Pt/β-NiAl diffusion couples. Based on the measured concentration profiles coupled with the local equilibrium hypothesis, the tie-lines between neighboring phases were determined. Two intermediate phases, Pt₃Al and α-NiPt(Al), are found to develop between the Pt and β-NiAl couples. The thicknesses of Pt₃Al and α-NiPt(Al) layers varies linearly with the square of annealing time, indicating that the growth of Pt₃Al and α-NiPt(Al) are diffusion-controlled. The calculated Arrhenius equations of the parabolic growth constants, κ, for the Pt₃Al and α-NiPt(Al) phases are obtained, respectively. The average ternary inter-diffusivities of the Pt₃Al and β-NiAl phases at different temperatures are determined from an integration of inter-diffusion fluxes calculated directly from the experimental concentration profiles.