A study of the formation of Nb3Sn layers at the junction of pure Nb and a Cu–Sn alloy is presented, and the effect of grain boundary diffusion on the kinetics of the formation of the layers is assessed. Data at 700–800 °C imply that the layer thickness increases with a t0.35 time dependence. This deviation from the usual t1/2 law for a process whose rate is limited by volume diffusion through the growing layer had previously been explained using analytical expressions for grain boundary diffusion into a semi‐infinite crystal. A numerical analysis of grain boundary diffusion with geometry appropriate to an intermetallic layer is presented, and the results imply that a fixed array of grain boundaries in the layer cannot cause appreciable deviation from a t1/2 law for most systems. The flux to the growth interface required to form the layer normally causes the diffusion processes in the layer to approach steady‐state conditions, which, in the case of fixed boundaries, precludes a large deviation from the t1/2 law. Scanning electron micrographs of the layer at various stages of growth show that the average grain diameter increases with time at an appreciable rate. The measured grain growth rate is used in a simple steady‐state layer growth expression to obtain a description which agrees with the experimental measurements. Other factors which can influence the formation rate of intermetallic layers are discussed.

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