The kinetics of boriding of Fe - Ni alloys containing 60, 70, 80, and 90 wt.% Ni at 1073 - 1373 K for 2 - 8 h in powdered mixtures with the use of Ekabor-3 boriding agent has been investigated. It has been established that a compact multiphase layer represented by nickel silicides, borosilicides, and borides is formed on the alloy surface. The hardness of the layer depends on the nickel concentration in the substrate. It is shown that the growth of the boride layer with time can be described by a parabolic dependence. The activation energy of the boron diffusion in Fe - Ni alloys is also a function of the nickel content.