Using Al/Fe/Al diffusion couples prepared by a diffusion bonding technique, the reactive diffusion in the binary Fe–Al system was experimentally examined in a previous study. The diffusion couple was isothermally annealed at temperatures of T=823–913 K, and then a compound layer of Fe₂Al₅ was observed to form at the interface. The observation indicates that there exists the parabolic relationship between the mean thickness of the Fe₂Al₅ layer and the annealing time. The parabolic relationship means that the growth of the Fe₂Al₅ layer is controlled by volume diffusion. A mathematical model for the reactive diffusion controlled by volume diffusion was used in order to analyze numerically the growth rate of the Fe₂Al₅ layer. Through the analysis, the interdiffusion coefficient D of Fe₂Al₅ was evaluated to be 5.93×10⁻¹⁶, 1.18×10⁻¹⁴ and 2.92×10⁻¹⁴ m²/s at T=823, 873 and 913 K, respectively. Expressing the temperature dependence of D as D=D₀exp(−Q⁄RT), values of D₀=2.34×10² m²/s and Q=276 kJ/mol were obtained by the least-squares method.