The reaction kinetics of eutectic SnPb solder on Cu were studied and compared in the liquid state at 200 to 240°C and in the solid state aged at 125-170°C. The ternary phase diagrams of SnPbCu, the morphology of intermetallic compound (IMC), and the kinetics of growth of the intermetallics were used in the comparison. The temperature difference between these two reactions is only 30°C, but the kinetics of reaction, as well as the morphology of IMC formation, are very different. The kinetics in the wetting reaction is four orders of magnitude faster than that in solid state aging. The Cu6Sn5 intermetallic morphology in solid state aging is a layer type, but it has a scallop-type morphology in the wetting reaction. The morphology strongly affects the kinetics. While the kinetic difference can be attributed to the difference in atomic diffusivity between the liquid state and the solid state, it is the morphology that determines the kinetic path in these reactions. We conclude that a fast rate of reaction, which leads to a high rate of Gibbs free energy change, controls the reaction, not the Gibbs free energy change itself.