In the binary Fe-Cr system, the gamma phase with the face-centered cubic structure becomes a stable intermediate phase sandwiched between the Fe-rich alpha(1) and Cr-rich alpha(2) phases with the body-centered cubic structure in the temperature range between T = 1,119 and 1,185 K. Within this temperature range, the reactive diffusion between the alpha(1) and alpha(2) phases was experimentally observed by Nishizawa and Chiba (Nishizawa and Chiba (1970) J Jap Inst Met 34:629). In their experiment, Fe/Cr diffusion couples were annealed at T = 1,123-1,163 K for up to 600 h. Although the gamma phase should be formed due to annealing, it was not recognized in any annealed diffusion couples. The mathematical model reported in a previous study (2004 Acta Mater 52:1193) was used to analyze their experimental results theoretically. In order to provide the boundary conditions for the analysis, the phase diagram in the Fe-Cr system was computed by a CALPHAD technique. The analysis indicates that the gamma phase is actually produced at the interface but its thickness is less than 1 mu m even after annealing at T = 1,163 K for 600 h. Thus, the gamma phase is practically invisible under their experimental conditions. The analysis provides the experimental conditions for the gamma phase to grow to observable thicknesses.