The kinetics of the interfacial reaction of a thin layer of Sn sandwiched between two pieces of Ag foil has been investigated at temperatures of 260 °C, 300 °C and 340 °C. A time dependence of the form t1/n with n = 3 was obtained for the kinetics of both the consumption of the Sn remaining and the thickening growth of the Ag3Sn scallops formed between Sn and Ag. Such a result can be explained well using the model of grain boundary/molten channel-controlled growth of intermetallic compounds. In this case, the diffusion of Ag atoms through the molten channels existing between the previously formed Ag3Sn scallops is the controlling mechanism for the kinetics. We also report here the derived kinetic constants including reaction constants and the associated activation energy for guiding the practical transient liquid phase soldering of the Ag–Sn–Ag system.