The tracer diffusion of iron in 18−8 stainless steel has been measured over a wide temperature range by the surface activity decrease method, using radioactive Fe55. The results show that the lattice diffusion coefficients can be described by the equationD=0.58 exp(−67 100/RT) cm2sec−1. The effect of variations in grain size has been determined, and it has been shown that when the grain size is small, grain boundary diffusion contributes appreciably to the over‐all diffusion process, this effect decreasing with increasing temperature.