A model for ion nitriding is proposed which is based on the formation and diffusion of vacancy‐nitrogen ion pairs. A theoretical expression is developed for the local effective diffusion coefficient of the combined vacancy‐ion pairs which explains the accelerated case hardening associated with ion nitriding as compared with conventional nitriding. Ion‐nitriding experiments on AISI 304 stainless steel were performed in a magnetic field to increase the number of vacancy‐ion pairs. The experiments confirmed the theory, showing that in the current density range 100 –1000 A m−2the local effective diffusion coefficient is proportional to the current density.