AbstractInterdiffusion measurements of Nb and Ta in Fe base alloys have been made. Dilute alloys containing up to 0·6 wt-%Nb or 0·56 wt-% Ta were prepared. Diffusion couples of these alloys against pure Fe were made by spot welding. After vacuum annealing, diffusion profiles were obtained using electron probe microanalysis. The diffusion parameters were determined at one standard deviation using the Grube method. In austenite, interdiffusion of Nb is in good agreement with previous tracer diffusion data. The diffusion rate of Ta in austenite is approximately six times that for self-diffusion of Fe. Inferrite, diffusion of Ta produces a higher value of activation energy than other transition metals. Attempts to measure the diffusivity of Nb inferrite failed owing to the different levels of concentration and the presence of an intermetallic phase. However, a solubility of Nb inferrite of less than 0·24 wt-% is reported.MST/1162